I don't wave unless I know someone, if I do it's a thumbs up.
I always thought it was stolen from farmers, you know every damn farmer in a pickup, on a rural road, that waves at you as you pass head on. I always wave at the farmers, never know when they'll be answering the door when I ask permission to hunt. I've ridden since the mid 60s, I don't remember this biker wave appearing till the last decade.

Every normal man must be tempted, at times, to spit on his hands, hoist the black flag, and begin slitting throats. H. L. Mencken