A hammer loop is a strip of fabric, about an inch wide, from the left side seam to the back pocket of a pair of carpenter's overalls. It incorporates a half-twist to make it stand out from the hip just a bit so you can put your hammer in it, head side up.
However, as devotees of Norm Abram can tell you, the hammer is obsolete. Real carpenters use hydraulic nail guns. Only weekend woodworkers, those rank amateurs, use actual hammers these days. But they do their DIY in old blue jeans, which of course do not have a hammer loop.
So who has a hammer loop any more?
Infants, that's who. Every pair of overalls I have seen in Baby Gap comes equipped with a hammer loop. The well-dressed tots, who cannot walk, much less hit a nail, have somewhere to put their hammers.
It's becoming the sartorial equivalent of an appendix.