An idiom is basically a "saying", "expression" or "figure of speech" in a certain language. It's meaning often doesn't directly relate to (and is not derived from) the literal words used to construct the idiom, and sometimes the idiom may be gramatically incorrect.

And no, just because you didn't understand the above definition from the literal words used to construct it, it doesn't mean that the entire previous paragraph is an idiom, idiot.

Idiot: "a foolish or stupid person" ¹

As you read some of the most common idioms below, with their meanings, you will understand what I am talking about.

Some of the idiom definitions below are taken from The Free Dictionary.

To hit the roof / blow your top - to become very angry suddenly. eg Sharon hit the roof when she discovered that little Johnny spent all of his birthday money on lollies.

To get your marching orders - to be given the sack, to be let off.

To receive a taste of your own medicine - to do the same bad thing to someone that they have often done to you, in order to show them how unpleasant it is. Baxter the bully received a taste of his own medicine when a group of nerds got together to beat him up.

To burn the midnight oil - to work very late

To give a person a piece of your mind - to speak angrily to someone because they have done something wrong.

To be over the moon - to be very happy

To be in never-never land - to be day dreaming or "to be in a different world" (note: not a reference to Michael Jackson's old residence)

To let the cat out of the bag - to tell people secret information, often without intending to. eg Ryan let slip that he had a crush on Mary, he let the cat out of the bag and now everyone knows.

To cry crocodile tears - to cry fake tears.

To be firing on all cylinders - to be doing very well or "operating as powerfully and efficiently as possible". eg Ian Thorpe was firing on all cylinders at the swimming events in the Sydney 2000 Olympic Games.

To look daggers at someone - to look very angrily at someone. Donna looked daggers at Richard when she overheard him telling his friends about their "undercover antics". (He quickly changed the subject.)

To let sleeping dogs lie - to not talk about things which have caused problems in the past, or to not try to change a situation because you might cause problems.

To be dressed to kill - wearing clothes that are intended to make people notice you. (James Bond is literally dressed to kill - pun intended). This one is similar to that ad for a hair product with Julia Louis-Dreyfus: "She's gonna stop traffic!"

To turn over a new leaf - to start fresh.

To rub salt in the wound - to make a difficult situation even worse. (Note rubbing salt in the world actually heals the wound, but it is very painful). eg Losing the game was bad enough. The other team rubbed salt in the wound by gloating while accepting the trophy.

To be at sixes and sevens - to be confused or badly organized

Not to have a leg to stand on - to be in a situation where you cannot prove something

To drink like a fish - to drink alcohol excessively

At the drop of a hat - immediately, at the slightest signal. If you need help, just give me a buzz, i'll be there at the drop of a hat.

To live from hand to mouth - to live in poor circumstances, day by day.

A bird in the hand (is worth two in the bush) - "something that you say which means it is better to keep what you have than to risk losing it by trying to get something better"

To be packed like sardines - to be packed very tightly, eg The bus was very crowded, we were packed like sardines! 

Say What You Mean !?! Challenging Idioms for English Language Learners

This idiom has really confused this idiot. If you have your cake, why wouldn't you eat it too? What, are you just going to look at it? It's there to be eaten for goodness sakes!

The intended meaning behind this figure of speech can probably be best explained by using (you guessed it) some more idioms! You can't have the best of both worlds. If you eat your cake, you can't have it as well (it is gone). You can't have it both ways!

Example: A man who is cheating on his girlfriend is having his cake and eating it too.

Solution: If you can't have your cake and eat it too, eat someone else's cake. (But then that cake will be having itself and eating it too, so that may not work after all, unfortunately)

This idiom refers to a place being very small. If you were to hold a cat by the legs and swing it around in an arc, it would basically hit every wall. (Please do NOT do this though - pussies are nice and furry... and for the most part harmless)

Click here for a humorous & more detailed analysis.