Idioms are common words or phrases that make sense in the U.S., but would probably have no meaning to a beginning speaker of English. They're also difficult to translate into another language.
For example, I might say he bought the farm or went toes up or bit the dust in place of saying he died.
Idiomatic English can also mean that a phrase is said using particular words, such as she talked down to him, which is not the same as she talked under him.
We try not to end a sentence with a preposition, but because of idioms it's okay for me to write I dressed up.
If your characters don't know idiomatic English, they might be from a country other than the United States, or they might not use idioms correctly for other reasons. Idioms can be an indicator of familiarity with correct English.
A few idioms that are often misused, according to Hodges' Harbrace College Handbook, include:
according to the plan [not with]
bored by it [not of]
die of cancer [not with]
independent of his family [not from]
happened by accident [not on]
jealous of others [not for]
Someone once told me that we should say a person has a flu rather than has the flu because different strains of influenza exist.
Not true. Has the flu is idiomatic English.
Have you heard idioms misused?
Infinities of love,