Thumb Rack

They're the dimples on the lower back where you can rest your thumbs when hitting it from behind.

Did you see that girl when she bent over to tie her shoes? Yeah she had a nice set of Thumb Racks.

Thumb Rack

The dimples located on the lower back used to rest the thumbs while hitting it from the back.

While Tiffany was bent down tying her shoes, Alec shouted" Those are some nice Thumb Racks!"