You know when someone puts their hand to their face,
perpendicular to the face itself... and they hook their thumb under their nose and waggle their fingers as a method of ridicule?
That is called cocking a snook.
If I cock a snook at you, I waggle my hand with thumb hooked under nose, at you.
I hope you've all got that, it's pretty hard to describe, but I think you know what I mean.