As a lifelong Anglophile and a recent newcomer to London, I can understand America’s burgeoning love affair with British English. But even with the spike in usage of Britishisms, there are still a number of words and phrases that can baffle even the most pretentious BBC America fans. Next time you’re in London, keep these translations to hand—or as the Yanks would say, nearby—and you’ll be just fine.
1. Knock up: To wake up. Don’t freak out if your flatmate says he will be sure to knock you up in the morning.
2. Pants: Underwear. Be careful not to compliment your friend’s new pants, or she will be very confused. Trousers or slacks are what you wear over your pants.
3. Take the piss: To take advantage of; to ridicule. This is one of the more unattractive British phrases that show up frequently in conversation.
4. Bum bag: Fanny pack. For your own sake, don’t say “fanny pack.” (Come to think of it, don’t say “fanny” at all.)
5. Poncy: An especially negative version of the word “posh.”
6. Plaster: A band-aid.
7. Whinge: To whine.
8. Cash point: An ATM.
9. Car park: Though it sounds more like an auto show, it’s just a parking lot.
10. Garden: Backyard. A front yard or lawn is referred to as the front garden.
11. Accident and Emergency: An emergency room or trauma center; commonly referred to as the “A&E.”
12. Pot: Carton or container, as in, “I had a yoghurt pot for breakfast.”
13. Sex pest: Though it sounds like some unpleasant disease, a sex pest is more akin to a sexual predator or someone who sexually harasses others.
14. Sign on: It has nothing to do with AOL—it actually means to sign up for welfare.
15. The dog’s bollocks: If something is the dog’s bollocks, it is excellent.
16. Tramp: Homeless person. You can still get upset if someone says you dress like a tramp; you’ll just be upset for a different reason.
17. Rude boy: Thug or delinquent. A rude boy in England probably has no special affinity for ska music, unlike the rude boys in the U.S. and Jamaica.
18. Jumper: Sweater or pullover.
19. Mucky pup: Messy person.
Kathleen Elise is a graduate student at University College London..
Source- http://mentalfloss.com/article/12843/19-confounding-discrepancies-between-american-english-and-british-english