the definition of suck mah d means for one to get down on their knees and suck one's d (dick). Suck mah d can either be feminine or masculine. You can say suck mah d in most context's either when your pissed of or when someone is being a douche bag. Suck mah d can also be a spur of the moment kind of saying, kind of like hello or goodbye.
Boss: Hey you, know your role!
Employee: Well how about you suck mah d!
Person 1: "Exscuse me I need to get by"
Person 2: "Suck mah d bitch"
Person 1:"Hey whats up"
Person 2: "Suck mah d"
Person 1: "You are very hostile"
PersoN 2: "Shut it or suck mah d