A person with messy hair that they haven't brushed in 3+ weeks. (maybe they don't wash it either) it's so gross that birds lay eggs in it thinking it's a nest. kashkoosh guys often have a kashkoosh moustache or beard as well.
Person 1: Ew! That guy's hair looks awful.
Person 2: Right?! Must be a kashkoosh.
Get the kashkoosh mug.