This is a category that falls under the area of historical context with respect to the use of language itself within a culture (i.e. extra biblical-usage). It also may be used for canonical context usage (i.e. term combinations that take on extra meaning from their usage in Scripture alone).
Questions should include an aspect about Hebrew, Greek, or Aramaic word groups (i.e. a phrase or clause) found in Scripture that may have had an historical, culturally idiomatic meaning that is itself distinct from the mere definitions of the words themselves because of the combination of those words.
Terms are often combined in cultural usage, the combination of which tends to have some unexpressed (non-explicit) idiomatic meaning associated with it that may or may not be directly related to the actual terms used.
An example in English would be "raining cats and dogs," which means "raining very heavily." The literal definitions for cat and dog have nothing to do with the combined expression itself (though there is speculation about the association), and one not familiar with the historical and cultural use of the phrase may be confused by the odd expression.
A Biblical example would be Dt 21:17, which uses "Beginning of his strength" to refer to a firstborn child.
Other idiomatic expressions may arise by them having elided words from what was a longer, commonly understood expression, and so the shortened expression is expected to be understood in light of one being aware of what was elided.
NOTE: Generally this category should not be used for single word discussions or definitions, as such can generally be found in any lexicon on the language (including figurative or idiomatic uses).