keep something under your hat = don't say anything to anyone: "I've got something to say to you. But keep it under your hat - it's not public knowledge."
take your hat off to someone = admire someone: "I really take my hat off to people who work full time and study at the same time!"
tied to his mother's apron strings = someone (normally a man) who does what his mother tells him: "He didn't want to come out last weekend, because his mother disapproves of us. He's really tied to her apron strings!"
keep something up your sleeve = keep something hidden for later: "We've been negotiating my new pay and conditions, but I've kept the other job offer up my sleeve for the time being."
all talk no trousers - someone who talks a lot but doesn't act: "I know he told you that he would get you a limousine for the wedding. Don't believe him, though. He's all talk, no trousers."
who wears the trousers? = who has the power in a relationship: "What do you mean, she won't let you come out with us? Who wears the trousers in your house?"
pull your socks up = work harder: "You'll have to pull your socks up if you want a promotion next year."
it will blow your socks off = very hot food: "This is a fantastic stir-fry - it's hot enough to blow your socks off!"
hot under the collar = upset or angry about something: "He gets really hot under the collar about cruelty to animals - he can't stand seeing animals suffer."
it's pants (UK slang) - rubbish: "What did you think of the film?" "Pants!"
get something under your belt - achieve something: "I'm really glad I passed the driving test. Now I've got that under my belt, I can relax for a little while."
belt up = keep quiet: "What's all that noise? Just belt up, would you? I can't hear myself think."
below the belt = unfair: "You know he's really sensitive about the accident. I think it was a bit below the belt to mention it."
the boot's on the other foot = your opponent now has the advantage: "Now that she has been promoted, the boot's on the other foot! You should watch what you say from now on."
get your skates on = to hurry up: "Get your skates on - we're late as it is!"
have the shirt off your back = to steal all you own: "He asked you for how much rent? He'd have the shirt off your back, if you let him."
in only the clothes he stood up in = to only possess what you wear: "After the fire, they were left with only the clothes they stood up in."
get shirty = become angry with someone: "Don't get shirty with me! I'm only reporting the new rules."
skirt around the issue = not talk directly about something: "They skirted around the issue for a while, then got down to the real business."
cloak and dagger = mysterious: "Who's arranging the party? I don't know - it's all very cloak and dagger at the moment."
give someone a dressing down = tell someone off / reprimand someone: "He gave the whole department a dressing down after they failed to meet their agreed targets."
dressed to the nines / dressed to kill = dressed up: "Where are you going, dressed up to the nines?"