Green’s Dictionary of Slang

s.a.b. n.

[abbr.; SE social + airhead n. (1) + bitch n.1 (1a)]

(US campus) a social airhead bitch.

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[US]Eble Campus Sl. Nov.
[US]Eble Sl. and Sociability 37: Two kinds of shortening reduce words to letters. The dominant type in college slang is initialism, which names the individual letters: [...] SAB (social airhead bitch).