Idiomatic expressions are slang expressions. Idioms are simply expressions that have a completely different meaning than the individual words that make up the expression. For example, the phrase add up is an idiom that has the meaning of consistent, as shown in this example:
When I asked the teenagers where they had been, their stories did not add up.
The English language has many idioms. Using them in written material, such as technical documents, is generally not recommended.
Some of the commonly used idioms, along with their meanings, are listed below.
As easy as pie — simple
Antsy — restless
Best around the bush — avoid an issue or question
The bottom line — most important piece of information
Go with the flow — take things as they come
Jump the gun — act hastily
Keep an eye on — watch out for
Leave well enough alone — Take no action
Make a mountain out of a molehill — overreact
Once in a while — occasionally
State of the art — up-to-date
Under the weather — not well