idiom n. An expression conforming or appropriate to the peculiar structural form of a language; [...] an expression sanctioned by usage, having a sense peculiar to itself and not agreeing with the logical sense of its structural form: the term "red herring," an idiom meaning "false trail," is used of something which is neither red nor a herring.

Idioms generally pertain to one of our senses:

Idioms are often used when a speaker can't think of an original, understandable way of phrasing a concept. The idiom is usually an expression the speaker is familiar with:

• the speaker has heard it before
• the speaker has used it before
• the speaker understands the context

None of these is required, of course -- occasionally an idiom is used though nobody knows exactly where it comes from (e.g. "the whole nine yards," for which Ann Landers has recorded dozens of sources).

Idioms can be misunderstood, or missed altogether, if

• the speaker and audience don't have a common frame of reference

  Sports metaphors are typical here. To understand some (e.g. "step up") you have to be a sports fan; for others (e.g. "the ball is in your court") you may have to be a fan of a particular sport.

  Many metaphors come from the military as well -- you may need a military interpreter to help you understand them! Other sources are Westerns (e.g. "shootouts," "hired guns," and "shoot from the hip") and computers (e.g. "core dump," "foobar")

• there is background noise, or some other distraction, at either end
• the speaker can't get the idiom right

  Scott Adams, creator of Dilbert, correctly indicates the confusion resulting from idioms mixed up in the workplace. In a recent Dilbert Newsletter, he presented examples of confused idioms that included these (italics mine):

  • I am going to let you move around more, just to break up the mahogany.
  • We don't want to screw ourselves in the foot.
  • I think it's time to get our sleeves dirty.
  • You can't see the forest if you're barking up the wrong tree.
  • We don't want to go barking up a dead horse.
  • Your work is late, but that's neither hide nor there!
  • The best way to learn is from the school of Fort Knox.
  • ... Does anyone have a concern? Speak now, or hold your piece!
  • The Indians are nervous at Waterloo. [WHAT am I supposed to emphasize HERE?]
  • Those new salesmen are still green behind the ears.
  • Well, I'm just busier than a one-armed naked man.
  • My manager once said, "We triumphed over diversity."

Idioms can be taken literally

• by internationals (unless the idiom has an equivalent in the audience's primary language -- how often does that happen?)
• by individuals with learning differences (e.g. ADD/ADHD)

If you have any reason to believe your meaning is unclear, you will have to communicate this to the audience:

• ask if they need you to repeat/rephrase/explain a remark
• ask them to repeat it back (in their own words if there is opportunity)

References

The Dog Hause Idioms -- animal idioms
Dave's ESL Cafe -- Idioms
The Book of Cliches
http://www.dilbert.com/