Home › Words › U › underbrimunderbrimunderbrim means the underside of a hat's brim.EtymologyFrom under- + brim.nounThe underside of a hat's brim.e.g.“Several of the colored hats had underbrims of a contrasting color.” — 1917, Dry Goods Economist, volume 71, page 81:Definitions & examples from Wiktionary (CC BY-SA 3.0).