hip pocket 
noun
A pocket in the back of a pair of trousers.She lifted her chin defiantly, shoving her hands into the hip pockets of her jeans....
- ‘Okay, that's it,’ Jeremiah said, stepping back and wiping his hands on a rag he had tucked in the hip pocket of his jeans.
- He stuffed Marisa's bikini top into his hip pocket and continued forward.
Phrases
in someone's hip pocket