A statement to acknowledge someone being flirtatious, a possible sexual situation; or compliment good looks. Could also be used a joke to note a completely platonic situation being one where the parties had devious sexual thoughts.
"Wasif is macking ass. Never chin chillin ass."
"Katass is always macking ass, never chin chillin ass."
"You're macking ass today."
"Damnnnnn, macking asss bro!"