List of British Words Not Widely Used in The United States - Y

Y-fronts: men's briefs with an inverted-Y-shaped frontal flap; originally a trademark (US: jockey shorts/briefs; US slang: tighty whities)
yob, yobbo: lout, young troublemaker (origin: boy spelt backwards)
yomp: to move on foot across rough terrain carrying heavy amounts of equipment and supplies without mechanised support (Royal Marines slang popularised by the Falklands war, army equivalent is to tab). Also used informally for any walk across rough ground.
yonks: a long time, ages. "I've not seen her for yonks." (colloquial)