bolsillo
/bolˈsiʝo/ · A2 · noun · m
👖👛 bolsillo = bolsa (bag) + -illo (little): a little bag sewn into clothes
Bolsa means bag or purse, and -illo is a Spanish diminutive ending. A pocket is literally a small bag. The money sense follows the same logic as English: paying de su bolsillo means paying out of your own pocket.