Green’s Dictionary of Slang

1. a fig. tight hold with the mouth.

1975

J. Wambaugh Choirboys (1976) 295: Roscoe, this just might be the night I get you in a lip lock and shut you up for good.

2. fellatio.

1976		in National Lampoon July 77: A liplock on the Chisholm Trail is better than a hired hand.
1984		K. Weaver Texas Crude 81: Put a lip-lock on my love-muscle.

3. (also lock lip) a passionate kiss, kissing.

1976		P. Conroy Great Santini (1977) 447: Mr. Suave is hauling ass out to the beach for a little lock lip.
1994		D. Lehane A Drink Before the War 37: [I]f I crawled up on a cat or two kids in a lip lock, I wouldn’t be able to show my face for a month.
2005		Jennifer Archer Sandwiched [online excerpt] It’s the kiss that does me in. I can’t tear my attention away from their passionate lip-lock, from Bert’s hands kneading and caressing that tight, round, voluptuous butt.