He knows not a hawk from a hand-saw.

The saying describes someone as so lacking in judgment or basic knowledge that they cannot tell one plainly different thing from another. It is a sharp, often insulting way to question a person's competence in the matter at hand.

It may also be used hyperbolically when someone makes an obvious mistake in classification, recognition, or choice. The comparison does not require actual knowledge of birds or tools; its force lies in imagining a failure to distinguish unlike objects.

Use it to criticize conspicuous confusion or ineptitude, especially where a distinction seems obvious to the speaker. As it can sound contemptuous when aimed at a person, it suits humorous, argumentative, or literary phrasing better than tactful feedback.