Feel a bit under the weather
feeling slightly sick
Pulling someone's leg
joking with them
Busy as a bee
To be very active and working hard at something
HItting the nail on the head
Performing a task with precision
It cost an arm and a leg
It is very expensive
Once in a blue moon
rarely happens
Raining cats and dogs
Raining very hard
A dime a dozen
Something that is very common, not unique
Let the cat out of the bag
share information that was a secret
Take with a grain of salt
Don't take it too seriously
Piece of cake
a task/job that is easy to complete
You can't judge a book by its cover
Can not judge something based on its appearance
Fit as a fiddle
Being in good health
Like two peas in a pod
Two people who are always together
The ball is in your court
It is up to you to make the next decision/step
He has bigger fish to fry
He has more important things to take care
When pigs fly
Something that is impossible
Sleeping like a log
Sleeping very soundly
Don't count your chickens before they hatch
Don't count on something happening until after its already happened
Biting off more than you can chew
Taking on a task that is too much to handle
Best thing since sliced bread
a good idea, invention or plan
Getting a taste of your own medicine
Being treated how you have been treating others
Run like the wind
to run very fast
At the drop of a hat
do something instantly without hesitation