H&H is a term used for people who are in a "friends with benefits" relationship with their partner. The acronym stands for "hang out and hook up", which means the couple went for a casual date followed by intercourse.
Dave: Knock Knock.
Clement: Who's there?
Dave: Europe.
Clement: Europe who?
Dave: That's not a very nice thing to call me.
Clement: L.S.V.T.I.W.M.N.T.W.I.H.O.H.M.N.V.E.T.D.T.S.C.F.T.N.K.T.T.A.T.T.R.O.T.H.T.B.D.M.D.T.A.C.T.T.M.A.O.G.H.H!