the.com/belt loop

means A small fabric loop sewn onto pants to hold a belt against the waistband so it stays put instead of sagging or riding up.

from A plain functional compound — 'belt' (from Old English 'belt,' itself from Latin 'balteus,' a sword-belt or girdle) plus 'loop' (origin uncertain, likely from a Scots or Northern English word for a bend or fold in rope). The garment feature itself is a 19th-into-20th-century tailoring development, arriving once belts overtook suspenders as the fashionable way to keep trousers up — the loops were simply invented to give the belt somewhere honest to hold on.