To get with a girl or to get with multiple girls.
No, this does not mean to make out with a hot girl but if you are "mackin' a biddy" or "mackin' biddies" then that is definitely an option depending on where things may lead with your biddy or biddies.
*Biddies are also known as "fly honeys"*
Bob: So what did you do over spring break, man?
Andrew: Oh I was mackin' biddies.
Phil: What is Andrew doing?
Terrence: Mackin' biddies.