"off the top of someone's head"
from the knowledge you have in your memory
"to cost an arm and a leg"
be extremely expensive
"to beat around the bush"
to talk about lots of unimportant things because you want to avoid talking about what is really important
"to be under the weather"
to be ill or feel ill
"once in a blue moon"
very rarely
"to bite the bullet"
to force yourself to do something unpleasant or difficult
"to get cold feet"
to feel too frightened to do something that you had planned to do
"a piece of cake"
something that is easy to do
"to put the wind up someone"
to make someone feel worried about their situation
"to cry over spilled/spilt milk"
to regret something that has already happened or cannot be changed
"to have your head in the clouds"
to not know the facts of a situation, to daydreaming and not paying attention to what is happening around you
"to be on cloud nine"
to be extremely happy and excited
"to hit the nail on the head"
to be accurately right about something
"to be in two minds"
to be unable to decide between alternatives.
"to split hairs"
to argue about small details of something
"at the drop of a hat"
immediately, without any hesitation
