Just sitting around doing nothing in particular. The act of crustin tends to take place after you get back from smokin a bunch of blunts. Usually involves masturbating, sleeping, and/or eating.
(1) Yo, what are you doin?
(2) Just crustin for a little while before I go out.
(1) Oh...call me when you decrustify.
see: hottest thing "under the sun"
Whoa, check out Crustin! What a HUNK!
(v)Meaning a crusty but desirable MAN (n) very sexy boy
Would you like to talk to Crustin over there, hes a really sexy boy
A Crustin is a kid who wears the same sweater everyday, often with a mismatched Yankees cap, maybe even a women's one. He also probably sports a classic bowl cut and listens to poser music, like Lil Wayne. A crustin is also said to have very little athletic talent or athelticism, but often tries as hard as he can.
Kid A: who's that kid in the yankees cap listening to lil wayne?
B: whatta crustin