Ever heard the expression "In like a dirty shirt"?

Question

I was talking with someone who had never heard that expression, so I googled it and came up with only 340 hits. Is this an uncommon expression now?

Answer 1

"Is this an uncommon expression now?"

I guess it must be, I've never heard it.

I don't think I "get it", either.   I googled it too, and I assume from the contexts I saw there that being "in like a dirty shirt" means "a sure thing", "a done deal", or something like that ... Why? :-)