Rumble in the Sack basically means to engage in the act of sexual intercourse. This can be with a man or woman. Rumbling in the Sack mainly means having sexual intercourse in a bed of sorts. This isn't a phrase used often as it is quite an old phrase. It is mainly used for classroom situations.
Holly: Hey Amber!
Amber: Oh, Holly! I haven't spoken to you in a while!
Holly: Yeah! I bet you were too busy having a Rumble in the Sack!
Amber: Yeah I was!
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