to kiss a partner passionately or what would be considered inappropriately in public (linked with PDA)
similarities to the meaning of making out however mackin' is considered to be early classic American slang
Jane: hey where's Shelley today?
Rachel: totally mackin' out with Martin behind the bleachers
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!"