E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Hawk and Handsaw.
I know a hawk from a handsaw. Handsaw is a corruption of hernshaw (a heron). I know a hawk from a heron, the bird of prey from the game flown at. The proverb means, I know one thing from another. (See Hamlet, ii. 2.)