What is an idiom?An idiom - is an expression, word, or phrase

that has a figurative meaning that is understood in regard to a common use of that expression that is separate from the literal meaning or definition of the words of which it is made.

E.g. in Croatian: mačji kašalj - something that is very easy

as blind as a bat Not able to see properly; blinde.g.  He lost his sight in an accident and is as

blind as a bat.

as sick as a dog very ill e.g. She was as sick as a dog after eating the

cake.

as stubborn as a mule to be very determined not to change your

decision or opinion about somethinge.g. You won't get him to change his mind -

he's as stubborn as a mule.

by shank's mare by foote.g. My car isn't working, so I'll have to travel

by shank's mare.

a cat nap a very short sleepe.g. a catnap left me refreshed enough to face

the rest of the day

a cock-and-bull story a silly story that is not truee.g. Don't give me that cock-and-bull story. I

asked for an explanation, and all I got was your ridiculous cock-and-bull story!

curiosity killed the cat Being curious can get you into troublee.g. Jill: Where did you get all that money?

Jane: Curiosity killed the cat.

dog-eat-dog worldRuthlessly competitivee.g. a dog-eat-dog society.

get (someone`s) goat to irritate someonee.g. I'm sorry. I didn't mean to get your

goat. Jean got Sally's goat and Sally made quite a fuss about it.

hit the bull´s-eyeto achieve the goal perfectly. e.g. Your idea really hit the bull's-eye. Thank

you! 

hold one`s horsesWait! e.g. Tom: Let's go! Let's go! Mary: Hold your

horses.

in two shakes of a lamb's tailin a very short time; very quickly. e.g. Jane returned in two shakes of a lamb's

tail.

let sleeping dogs lie  Leave something alone if it might cause

trouble.e.g. Jill: Should I ask the boss if he's upset at

my coming in late in the mornings? Jane: If he hasn't said anything about it, just let sleeping dogs lie. 

let the cat out of the bag to reveal a secret or a surprise by accident. e.g.  It's a secret. Try not to let the cat out of

the bag.

a lone wolf a person who prefers to do things on their

own e.g. The typical role for Bogart was the

Casablanca character, a lone wolf, cynical but heroic.

look a gift horse in the mouth to be ungrateful to someone who gives you

somethinge.g.  I advise you not to look a gift horse in the

mouth.

rain cats and dogs  to rain very hard. E.g. It's raining cats and dogs. Look at it pour! 

top dog the leader or chief of a groupe.g. He is the the top dog of their group.

when the cat's away, the mice will play

When no one in authority is present, the subordinates can do as they please. 

e.g. Jill: You shouldn't be reading a novel at your desk. Jane: But the boss isn't here. And when the cat's away, the mice will play.

a copycat a person who imitates or copies anothere.g. Stop copying my answers. You copycat!

work like a horse  to work very hard. e.g. You work like a slave in that kitchen. I'm

too old to work like a horse. I'd prefer to relax more.

make a monkey out of (someone) to make someone look foolish. e.g. John made a monkey out of himself while

trying to make a fool out of Jim.

cast pearls before swine to waste something good on someone who

doesn't care about it.  e.g. To sing for them is to cast pearls before

swine.