It is like fuck buddies but in a nicer way. Usually two people who have been in a relationship but it as not worked out. They still have some sort of sexual feelings for the person though, So become play mates. As in having sex, fingering etc.
Me and jessie didnt work out so now we're play mates
14 year old girls who sneak out to parties and only take about two cruisers yet thats still enough to get them wasted and having them put out to anyone capable of getting a hard on down at the back fence
Jason: Got a root last night
Greg: Me too, I banged a playmate
Jason: Yea, they are fucking easy as, and all they need is one drink to get them messed up Greg: Yea but they are still a shit root!