When an arsehole is not correctly wiped and after a few hours the left over excrement goes dry and crusty, thus leavin crusties attached to bum hair.(note after exercise sweatmay cause these to dissolve and let off an unpleasant smell)
He was in a rush and did not wipe his ass properly, after his nightly training he realised a real bad smell was comin from his rear end. and thn said to his friends "oh shit i hav crusties"
Congregating wherever aged proto-dance acts play at summer festivals, and typified by the "Dog-on-a-string"; Crusties are aging psychadelia fans, acid burn outs, or just vegans who don't like to wash their hair (or their dog).
Stumbled into the Ozric Tenticles tent, tripped over a dog on a string and had mexican mushrooms shoved up my jumper by a unwashed hippy. I love the smell of crusties in the summer.