Green’s Dictionary of Slang

m.l.a. n.

[abbr. massive lip action]

(US campus) passionate kissing.

[US]Eble Campus Sl. Mar. 4: MLA – making out, kissing.
[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: [...] MLA (massive lip action) ‘passionate kissing’.