in the sack
(idiomatic, slang) In bed (either literally or figuratively), especially with reference to sexual intercourse.
I'm pooped—I'll be in the sack if anyone needs me.
Her girlfriend was amazing in the sack.
In high school I had sex with girls quite a few times. They were straight women who I convinced to jump in the sack with me. Portia de Rossi
He was... a womanizer, he wanted to be in the sack with everybody. Lauren Bacall
Has anyone ever told you," I said, "that you coalesce reality?" "No. They only say that I'm good in the sack." "They are accurate but limited," I said. "And if you give me their names I'll kill them. Robert B. Parker
Don't buy a cat in the sack. Hebraic Proverb