(n) frend' sest
To "fool around" with a person or number of people who is/are part of the closely-knit group of friends one has in his/her life. This act is often seen as a taboo, as it is prone to adding a negatively dramatic twist in the social lives of the interrelated people in said socially interacting group.
"Holy crap, John had friend-cest with Jenny!"
"OMG!@! what a bitch!"
"Let's hate them!"