1. An initialism for "other people's property." Used in informal written communication. A: "I'm sorry, dude. I lost your book." B: "WTF, man? Learn how to take better care of OPP!"

2. vulgar slang Sexual encounters with "other people" (indicated by the first two initials) who are not one's partner; sexual encounters that constitute cheating on the part of one or both people involved. Coined in the 1991 song "O.P.P." by hip hop group Naughty by Nature, the lyrics of which imply that the final "P" can alternately stand for "pussy," "penis," or "property." Being "down with OPP" means being open to such encounters. I sing along at the club, but no, I'm not down with OPP.