Cars. In the UK, only the luxury car market have automatic transmission - in other words the Jaguars, Rolls Royces and Bentleys of the world. Most cars have manual transmission. This is because our roads aren't straight. As a consequence all learner drivers have to learn how to drive using a car with manual gears. I was told that in the States this is referred to as 'learning how to drive stick.' In the UK, asking your driving instructor whether he could teach you how to drive stick may cause potential embarrassment...
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Crossing the road. In the UK we love our cute fluffy and feathery friends. So much in fact that we name our road crossings after them. We have pedestrian walkways that have broad black & white stripes (like on the cover of 'Abbey Road' by the Beatles) which we call 'Zebra Crossings'. We also have crossings akin to yours with the 'walk/don't walk' signs on them which have a little red man standing still and a little green man walking. These are illuminated when you are supposed to stay where you are or walk respectively. For some inexplicable reason this is called a 'pelican crossing'. As for the little green man flashing...
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Git. An undesirable and miserable person. Between 'sod' and 'bastard' on the 'are you going to get your head kicked in?' scale.
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Jock. In the US, big guys who like sport, women and acting macho. In the UK, a Scottish person who probably also likes sport, women and acting macho but in a Glaswegian (i.e. from Glasgow) accent. Which is probably more scary since a lot of people have difficultly understanding them...
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Lemonade. In the US, non-fizzy fruit drink possibly made from lemons that we Brits call 'squash'. Our 'lemonade' is fizzy, akin to your pop or soda (depending on what part of the US you are from.) I was most disappointed when I found this out for the first time in a US cinema...
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Hotels. In the UK the floors in a hotel are numbered ground floor, first floor, second floor etc. In otherwords the first floor is the second floor, the second is the third and so on and so on. In the US, you have a more sensible numbering system. A good thing to note if you are a US bell-boy(UK)/bell-hop(US) looking for Take That's (screaaaaammmmm!) suite on the eighth floor in a UK hotel. (BTW Just follow the detritus of fluffy toys and soggy knickers (cv)...)
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Jelly & Jam. In the UK, jelly is either the stuff you US-types call jello or a seedless preserve made from fruit, sugar and pectin. To confuse things further, fruit preserves are generically called jam over here too. Hence, if you were in an English restaurant enjoying a piece of bread with peanut butter and fruit preserve on it you would be eating 'a peanut butter and jam sandwich.' BTW, I used to enjoy peanut and jelly sandwiches when I was little in the UK sense of the word... Sloppy, but very nice.
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Khaki. In the UK a light beige colour. In US khaki can also be green when referring to army fatigues which are generically known as 'khaki'.
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Knickers. A similar problem to 'pants' (cv). In the US they are knee-length trousers like what the Brits call 'breeches'. In the UK, they are the things that go underneath. Typically British men wear pants under their trousers and women wear knickers, unless of course, you are a Tory (Conservative) MP and then anything goes... Also NORWICH was an acronym used by service personel during WWII for '(k)Nickers Off Ready When I Come Home'. To be on the safe side when visiting the doctors it's best to keep your pants/knickers on...
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Spanner. You see that long metal object in your tool kit that you use to adjust bolts on your car? We call that a spanner, not a wrench.
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Stones. To you big rock things that geologists play with. To us also a unit of weight. 1 stone is equal to 14 pounds. Also, English pints show remarkable value for money compared to their US conterparts - 567ml compared to 430ml. Good thing to know when ordering beer.
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Warm clothing. In the UK we wear warm woolly upper garments during the winter which we call 'jumpers'. You call them 'sweaters'. Boring but true. Also a long woolly dress is called a 'jumper' in the US. I suppose both nations have the joke: What do you get if you cross a kangaroo with a sweater? A woolly jumper. Groan. Somebody carbon date that joke please...
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Wellies. In the UK a type of waterproof rubberised boot named after that Great Englishman, the Duke Of Wellington. You guys in the US would call them 'gumboots' or 'galoshes'. In the UK wellies are much beloved of Tory MPs with large country estates and farmer-types with sheep, particularly the 'Hunter' welly with the handy straps on the side.
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Link (as previously):
http://www.dur.ac.uk/~dgl3djb/~ukus.html