Good in the sack?

I was walking down a set of stairs recently when I over heard two women talking…

“blah blah blah blah”

“Yeah but I bet he’s good in the sack”

“blah blah blah blah”

I know they’re talking about sex right now, and I know what good in the sack means, but do I know what it really means?

What are the origins? This is my breakdown… I think it means if two people were tossed into a actually sack they would fool around and have some fun… Like two shrunk people in a confined space - or sack. That thought alone makes me laugh.

Anyone have any better ideas?