It would seem that there is a line at the beginning of Gilbert and Sullivan's Trial by Jury which goes, "be firm, be firm, my pecker." In England, it would further seem, pecker means something rather different than what it does here. It means courage. I'd very much like to know how that happened. I don't know about you, but I have a tendency to laugh unwillingly at even the slightest innuendo; I imagine that if I were to have seen Trial by Jury without foreknowledge, I likely would have been escorted out of the theater for being disruptive.
On the other hand, there's the perfectly innocent word fanny, which over here is how moms and Kindergarten teachers say butt. In England, however, it's rather a lewd term for the body part opposite the fanny on a woman.
Fit in England can be a slang term for good looking, where in the US, phat can mean the same. Here, pissed is a synonym for angry, whereas in England it means drunk. Here, crack is something you smoke, where there it's something you have at a party - fun.
I think in most parts of the US, people know the word oy as half the Yiddish expression oy vey, meaning woe is me. There is, I just learned, a movie called Oy Vey! My Son is Gay, which is maybe the best movie title I've heard in years. In countries from the British Isles to Singapore of all places, Oi is an exclamation meant to get someone's attention, like hey. Wikipedia says that the term, which is most commonly associated with the Cockney dialect, may have come from ancient Romans, but I suspect it's just one of those sound effects that every language just sort of has, like oh and ow. Oi is also the mating call of punk rockers the world over, likely due to the English working class influence on early punk.
Punk, by the way, started out meaning prostitute, came to mean hoodlum, and probably came to be associated with the genre because it's an insult people often threw at the people founding the movement. I've heard people use the word punk to refer to a state of feeling in ill health "I'm feeling punk today," which appears to be etymologically related to the Native American word ponk, meaning dust or ashes, making it not etymologically related to the music genre. I just noticed that a whole lot of words that describe bad things end in unk. Funk, flunk, bunk (as in bull)...
Knocked up in England can mean woke up, but here it's semi-impolite slang for pregnant, generally of the unplanned or unwed variety.
Ready for a fun one? In England, nark can refer to a spy or informant which goes all the hell the way back to Sanskrit, for nose. It is etymologically unrelated to narc, short for narcotics officer. Did your head just explode? Mine just exploded. I've always spelled it narc, and always thought that it came from narcotics officer. I wonder how many other synonymous homophones there are.