From Longman Dictionary of Contemporary Englishin the sackin the sackinformal in bed – used to talk about sexual activity I bet she’s great in the sack. → sack
Examples from the Corpus
in the sack• This time he did not withdraw it at once but stayed motionless, his arm deep in the sack.• Puss put some bran and leaves in the sack near where rabbits lived.• Not really so marvelous in the sack.• He always stopped and would try to get me in the sack again.• On the track or in the sack, Ayrton Senna remains Formula One's only true winner.• Nothing but sickness and dung remained in the sack of my torso.• I took the collar off, removed the stones, put the rest in the sack and took it to the gibbet.• Carla caught the two of them in the sack together.• He put them in the sack. 