Why was 'Jesus' spelt 'Jhesus' in Wycliffe's Bible?

Etymology of Jesus: From Middle English Jhesus, Iesus, from Latin Iēsūs, from Ancient Greek Ἰησοῦς (Iēsoûs), from Biblical Hebrew יֵשׁוּעַ‎ (yēšū́aʿ), a contracted form of יְהוֹשֻׁעַ‎ (yəhōšúaʿ, “Joshua”). The form יֵשׁוּעַ‎ (yēšū́aʿ) is attested in some of the later books of the Hebrew Bible (Ezra–Nehemiah), and translated as Jeshua or Yeshua in some English editions (the former appearing in the King James Version). The Greek texts make no distinction between Jesus and Joshua, referring to them both as Ἰησοῦς (Iēsoûs).

In the Wycliffe Bible (Middle English), the forms used are Jhesus and Jhesu (Wikitionary)

Jesus is from Latin and in Middle English, people inserted an intrusive h into words that were borrowed from Latin (the intrusive h was mostly used in the beginning of words such as "(h)oste" and "(h)onour etc). And apparently the h was silent so it didn't change the pronunciation. It's probably because of that.

See this page: http://homes.chass.utoronto.ca/~cpercy/courses/MESpelling.htm also http://scottkleinman.net/medlit/ReadingMiddleEnglish.pdf

I can't improve upon Andrew Leach's excellent answer but I'll still try my best. I may be right or wrong, take it with a pinch of salt.

The name Jesus came into English from the Latin Iesus, a Roman transliteration of the Greek Iesous. It had come into Greek from the late Hebrew or Aramaic Yeshua, which was a common name for Jewish boys at the time of Jesus’s birth. The letter J didn't exist yet at the time, it wasn't invented until the 16th century.

When was J added to the alphabet?

J is a bit of a late bloomer; after all, it was the last letter added to the alphabet. It is no coincidence that I and J stand side by side—they actually started out as the same character. The letter J began as a swash, a typographical embellishment for the already existing I. With the introduction of lowercase letters to the Roman numeric system, J was commonly used to denote the conclusion of a series of one’s—as in “xiij” for the number 13. [Dictionary.com]

What looks like a J in medieval texts is actually just an I. It's just that the letters u, i, v, w, m, and n were all written using a sequence of a particular short downstroke of the quill, called a minim. So Wycliffe spelt the name Ihesus (or Ihesu ¹).

Here's how a minim looked like:

The word minim itself would have been written using only minims:

Where did the h come from:

In Middle English, ie was a vowel (just like in Modern English e.g., in lie). So if they had simply used the Latin spelling Iesus, people would have mispronounced the name. The added h serves to force pronunciation of the I as a consonant (or at least as a separate vowel which then automatically turns into a semi-vowel y-sound when pronounced fast). Therefore, Wycliff used the h in order to indicate that he's using the i as a consonant.

I speculate that the addition of h used to fricate or affricate certain sounds. Most of the affricates and fricatives have the letter h in their spelling. See, for example, <sh>, <ch>, <zh>, <ph> (for /f/), <th>. Perhaps Wycliffe thought he could do the same with I (by adding h to it) and would change its sound from a vowel to a consonant but I'm not entirely sure (it's just a speculation).

How did the J get its sound:

Both I and J were used interchangeably by scribes to express the sound of both the vowel and the consonant. It wasn’t until 1524 when Gian Giorgio Trissino, an Italian Renaissance grammarian known as the father of the letter J, made a clear distinction between the two sounds. Trissino’s contribution is important because once he distinguished the soft J sound, as in “jam” (probably a loan sound), he was able to identify the Greek “Iesus” a translation of the Hebrew “Yeshua,” as the Modern English “Jesus.” Thus the current phoneme for J was born. It always goes back to Jesus. [Dictionary.com]

In Modern English, the tittle over j and i signal a change in either the sound or meaning of a character.

Both the spellings Jesu and Jhesu are also used in Havelok and Dave but pretty irregularly.

Line 540:

Tho was Havelok in ful strong pine.
Wiste he nevere her wat was wo!
Jhesu Crist, that makede go
The halte and the doumbe speken,
Havelok, thee of Godard wreke!

Line 595:

So ther brenden cerges inne.
"Jesu Crist!" wat Dame Leve,
"Hwat is that lith in ure cleve?
Ris up, Grim, and loke wat it menes!
Hwat is the lith, as thou wenes?"

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¹

^{The absence of a final s was an influence of Old French, the OED says Iesu represented the Old French objective form of the Latin Iesus, and that was the form that came into Middle English and was used for some 400 years.}

^{The spelling Iesus, representing the Latin nominative form, was rarely used in Middle English but became the regular English spelling in the 16th century, according to Oxford.} [Grammarphobia]

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References:

• How to read Medieval handwriting - Harvard University
• History of the letter J - Wikipedia
• Middle English (orthography) - Wikipedia
• The letter J - Dictionary.com
• Minim - Wikipedia
• History of the name 'Jesus' - Grammarphobia

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Another speculation:

I speculate that Jesus came into English from Latin. I've heard that in Latin there was a sound change from I/Y (/j/ sound) at the start of words to J (/dʒ/) around the second century. (Also explained in an answer to Why is the J in “hallelujah” not pronounced as /dʒ/, but as /j/? by herisson)

So Iulius Caesar became Julius Caesar.
Latin Iupiter became Jupiter.

I guess Yeshua was Latinised to something like Yeshus and became something like Jeshus in Latin and then came into English where it became Jesus.

This is pure speculation.

It may be a latinisation of the Greek spelling (ΙΗΣΟΥΣ) - the half-Greek, half-Latin nomina sacra IHS (or IHC) and XPS (as well as inflected forms like IHU, XRI) were often used in manuscripts, including those of Wyclif's Bible. Nomina sacra, like other abbreviations, are usually marked by a horizontal line above them.

For example, in MS Lich 10 (written in the late 1400s), Wyclif's translation of Matthew 1:16 ([... the hosebonde of ] marie; of whom ih̅c was boren), where ih̅c (note the horizontal line!) is usually transcribed as Ihesus:

See also the form Ihesu Xpisto in medieval Spanish, where even the chi and rho have been replaced by their Latin lookalikes.

Some manuscrips, e.g. The Hague, KB KA 16, around 1350, of Jacob van Maerlant's Der naturen Bloeme, write Ihesus in full: (D[aer] Ihesus omme storte siin bloet)

So, in summary:

• The fact that the spelling "Ihesus" is found in several, phonetically quite disparate, languages (often together with the clearly non-phonetical "Xpistus") makes me think that there must be a non-phonetical reason for this particular spelling. The "greek letter lookalike" hypothesis would fit the bill.
• "Ihesus" may not even be "Wyclif's spelling" at all, but a spelling chosen by some later transcriber as an expansion of the nomen sacrum IHC (which strengthens the argument that the "H" ultimately derives from the Greek letter ήτα)