1) An axe with a broad head, or alternatively, a kind of adze.
The OED has examples from 1352 and c.1400, and defines the implement as an axe with a broad head, formerly used in war and for hewing timber. Evidence in a Yorkshire carpenter's will points to a possible alternative meaning: 1505 a chipe ax othyrwise called a brod ax, York. The chip-axe was apparently a kind of adze and if that is sometimes the meaning of broad-axe it may imply that it had developed from 'broad-axe': 1394 j crescet, j knave & burdas pro j lecto, York
1400 a burdhax ... thixtill, York
1471 Meo apprenticio j wodax, ij brode axis ... and j thyxill, Harome. If the two terms were not connected in meaning, it may be that metathesis influenced some spellings.