Crundies are undies that are crusty from dried up semen usually from masturbation ejaculating into the underwear and leaving it there to dry. Crusty+Undies=Crundies.
When a person wears the same socks and underwear for more than 3 days in a row. Creating a filthy smelly individual. The socks and underwear are known as chundies.
Brett "What the fuck is up with Mikey, he smells like a Billy Goat"?
Matt "Ya he has been rockin those chundies for 5 days now".