“In Pittman County there was nothing in pants that was worth the trouble, take my word for it.” What does the idiom, “nothing in pants” really mean?

They were speaking of men............no man was worth the trouble.

An idiom is a group of words established by usage as having a meaning not deducible from those of the individual words.

It's kind of like "ants in your pants". Nothing, in your example, would denote empty.