Short form of query, and of question. A named abbreviation rather than a concept.
A single-letter clipping standing in for either query or question depending on context, both of which trace to Latin quaerere (to ask, to seek); the letter itself carries no independent history beyond the clipping.
Whichever of query or question the context calls for, held to one letter. A named abbreviation doing double duty, honest about carrying no history of its own beyond the clipping.