We'll be spending a couple weeks this summer in the UK (Scotland and London), then a week in Ireland. Is there any slang common in the United States that we be best avoided in either country? For example, 'fag' is commonly used in the UK for cigarettes, but it's considered a slur in the US.
Avoid the use of the word "fanny", as it is slang for, ah, a woman's front bits. Be very careful when referring to your fanny pack!
Also, if an attractive member of the opposite sex asks you if you have a "rubber", double-check to make sure there aren't any chalkboards around because you're probably being asked for an eraser.
I heard an "Irish Car Bomb" and "Black and Tan" are types of cocktails/drinks. These are not terms that would be understood in Ireland, nor terms that are particularly nice.
The Irish Car Bomb one is obvious. "Black and Tans" were a group of British merceneries sent over in the Irish War of Independence in 1920s which were not exactly very honourable military people. They are viewed by Irish people the same way African-Americans might view the Klan. Nike and Ben & Jerry's have made this mistake.
Your example of fag is technically correct, but remember that "fag" would also be understood as an offensive slur in the UK & Ireland aswell, mostly depending on context/tone.
Would probably avoid trying to make jokes about "bad teeth" and horrible food as well. Brits in general don't take to kindly to jokes like that. In fact, I'm currently living in the states and some of the clichés the Americans have about us I can't understand where they even came from.
Also, if someone calls you a "yank" they don't mean to be offensive. It's quite common in the UK to refer to the Americans as yanks.
There's not a lot you guys say that offends us in general. Don't make the mistake of calling an Irish, Welsh or Scotsman English or you'll probably be ruined for ignorance.
It applies to gestures too. If you're holding up two fingers to mean two (two pints, for example), be sure to do so palm out. Holding up two fingers palm in, especially if accompanied by an upward motion, is rather rude. (In the south of Europe it indicates a cuckold; in the UK and Ireland it doesn't have that specific meaning, but it's still rude.)
You may hear fanciful stories about how this gesture originated from the Battle of Agencourt. These stories are not true.
This may help you. copied from here: http://funnychutkule.com/3446/british-vs-american-english-words-differences/