The Cornish Pixies remaining unaffected by Peskipiksi Pesternomi.
Peskipiksi Pesternomi is a spell that, according to Gilderoy Lockhart, is supposed to round up pixies. It is first used in 1992 in Lockhart's second year Defence Against the Dark Arts class. It had no effect on the pixies that Lockhart had let loose in the classroom, or any other effects at all, presumably because he just made it up right on the spot, a fact that i heavily supported by his extreme lack of true magical talents. Hermione Granger had used an exceptionally clever Freezing Charm which, unlike Gilderoy Lockhart's spell, actually indeed worked. It truly is unknown if this spell does actually work under any known circumstances.
Suggested etymology
English pesky meaning "annoying", English pixie meaning "a supernatural being", English pester meaning "to annoy", English no for negative and English me for the first person pronoun.
The spell, when spoken, sounds like "Pesky pixie, pester no me!". This would literally mean "annoying pixies, stop annoying me!".