English Etymology From un- + customary. Adjective uncustomary (comparative more uncustomary, superlative most uncustomary) Not customary. 2021, Colson Whitehead, Harlem Shuffle, Fleet, page 288:He had kept it cryptic on the phone; that plus the uncustomary afternoon meeting would warn the jeweler that it was serious.Wikiwand - on Seamless Wikipedia browsing. On steroids.