What does P.E.K.K.A stand for?

From the Clash of Clans wiki:

The letters in P.E.K.K.A don't officially stand for anything in particular. Supercell held a Facebook contest on 22 August 2012 to see who could come up with the best written-out name for P.E.K.K.A as judged by the number of likes the comment received. The name "Perfect Enraged Knight Killer of Assassins" was the winner.

In addition to this, "Pekka" is a popular given name in Finland, where Supercell (developer of Clash of Clans and Clash Royale) is from. (Thanks to user @Jeffery Lebowski for pointing this out).