And here I am again with English idioms. This time without long funny stories, but several at once! Here goes:
Don't drink the water! — a fairly popular parting word to a traveler. When I was translating the Conchords, this phrase ate my whole brain. It turned out to be advice not to drink tap water far from home. Because a tourist's metabolism may not cope with a different bacterial background and the composition of water from a foreign water supply.
Donkey's years — a half-joking expression meaning a long, long time. Years as long as a (living) donkey's ears. Long ago the English simply said donkey's ears — also about a period of time as long as a donkey's ears. But in a number of accents ears and years sounded very similar, and over time the expression turned into donkey's years.
Rain check — a very popular idiom, usually rendered in Russian as "let's do it another time?" The meaning is very close, but there's a nuance. A rain check is a kind of "rain ticket". They were given to people who came to a sports match that had to be cancelled and rescheduled due to bad weather. With a rain check you could get into the rescheduled event. So the idiom carries not just an abstract "let's put it off till some day, maybe", but a quite specific "not now, but we'll keep the option".
K9 — as a kid I, like many, thought that in the Belushi film "K-9" "K9" was some kind of special police unit code. When I grew up, while watching films in English I suddenly discovered that it's actually a homophone. That is, K9 simply sounds like something else. If you don't know yet, guess for yourself what :)