Green’s Dictionary of Slang

m.r.a. n.

[reeb n.2 ; abbr. major/massive reeb action]

(US campus) unsociable behaviour.

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[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: [...] MRA (massive reeb action) ‘unsociable behavior’.