Unpossible / Impossible

Ngram shows a prevalent usage of impossible vs unpossible also in the past centuries. It appears that unpossible has always been a less common variant:

Unpossible:

• (Etymology) from Middle English unpossible, equivalent to un- ‎(“not”) +‎ possible.

• (now rare, nonstandard) Impossible.

• 1526, William Tyndale, New Testament, British Library 2000, p. 119:

  • And this is the. vj. moneth to her, which was called barren, for with god shall nothinge be unpossible.
• 1621, Robert Burton, The Anatomy of Melancholy, New York Review of Books, 2001, p.280:
  • ’Tis a hard matter therefore to confine them, being they are so various and many, unpossible to apprehend all.

Impossible:

• (Etymology) from Old French impossible, from Latin impossibilis, from in- ‎(“not”) + possibilis ‎(“possible”), from possum ‎(“to be able”) + suffix -ibilis ‎(“-able”).

• Not possible; not able to be done or happen.

• 1300 Cursor M. 14761 It es bot foli al þi talking, And als an inpossibile [Gött. impossible] thing.

• 1340 Hampole Pr. Consc. 6281 Swa witty and myghty es he Þat na-thyng til hym impossibel may be.

Un-versus In- from Word Wide Words:

• In general, words take un- when they are of English (Germanic) origin and in- if they come from Latin. (The forms im-, il-, and ir- are variations on in-.) Apart from that, there’s really no good guide to which one you should choose. You’re just going to have to stick to learning them by rote.
• If it’s any consolation to you, the battle between in- and un- has been going on for centuries, with sometimes one form winning and sometimes the other, which suggests that the problem has been troubling English speakers for a very long time. As an example, for several centuries English had both inability and unability, but the latter disappeared in the eighteenth century for no very obvious reason.
• Another is familiar from the American Declaration of Independence: “We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable rights ...”; these days, it’s inalienable (it should always have been, by the rule, since alien comes from the Latin alienus, of or belonging to another person or place).