Taking part in the locking of lips. Commonly known as smooching, snogging, sucking face, etc. Derived from the more well known, and certainly less interesting, "making out."
"Hey Vanessa, I like you. Wanna go mack out in the woods?"
"Did you see those two mackin' out in the hall yesterday? I thought I was going to projectile vomit it was so repulsive!"