It comes from the Anglo-Norman (the dialect of French spoken in England following the Norman Conquest). The word achatour, meaning a buyer or purveyor of provisions, first appears in the mid-thirteenth century as a surname, as in Robertus le Achatour. The earliest citation in the Middle English Dictionary for the word being used as an ordinary noun is in Chaucer’s Canterbury Tales (c. 1387), where it says in the General Prologue, lines 567–69:
A gentil Maunciple was ther of a temple,
of which achatours [vrr: acatours, a catour] myghte take exemple
For to be wise in byynge of vitaille.
(A manciple was a purchaser of provisions for a monastery, an inn of court (i.e., temple), or other organization.)
The verb doesn’t appear until the seventeenth century. The earliest citation in the Oxford English Dictionary, which can probably be antedated as the entry appears to date to the nineteenth century, is in Shakespeare’s As You Like It (II.iii):
He that doth the Rauens feede, Yea prouidently caters for the Sparrow.