An example of def used as an adjective is in the phrase "that music is def," which means the music is excellent.
adjectivedef·fer, def·fest Slang
Origin of defShort for definite
(comparative deffer, superlative deffest)
- (US, slang) very good (short for "definitive" or "definitely")
- (law) Abbreviation of defense.
This is the customary abbreviation of this term as used in case citations. See, e.g., The Bluebook: A Uniform System of Citation, Nineteenth Edition (2010), "Case Names and Institutional Authors in Citations", Table T6, p. 430-431.