Green’s Dictionary of Slang

(US campus) passionate kissing.

1981		Eble Campus Sl. Mar. 4: MLA – making out, kissing.
1996		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’.