One funny idiom is "put a sock in it," which someone says to a noisy person to make him quiet down. Another funny idiom is "as cool as a cucumber," which refers to someone who is calm and collected.
Some funny idioms that come from the British include, "Bob's your uncle," "all talk and no trousers" and "more holes than Swiss cheese." "Bob's your uncle" means that everything is alright, and it roughly translates to "and that's that." "All talk and no trousers" refers to someone who constantly talks about doing important things but never actually does them. "More holes than Swiss cheese" refers to something that has numerous problems or is lacking in significant areas.