J-Some

A threesome involving a couple and an outside third party, in which the couple has intercourse and then one member of the couple has intercourse with the third party, leaving the other member uninvolved.

Often used as an excuse for a member of a couple to have intercourse with somebody who is not their significant other.

“My girlfriend keeps asking for a j-some with her friend, I’m starting to think she just wants to be with her instead.”