Acre, the unit of land measurement, comes down to us from the Old English æcer. The word is common throughout the Germanic languages, and has cognates in other Indo-European languages too, like the Latin ager and the Greek ἀγρός ”field," and the Sanskrit ajra ”plain, open country.” The modern spelling is influenced by the Norman French version of the word.
The exact definition of acre has varied over the centuries, and originally it referred to the amount of land a team of oxen could plow in one day, as recorded in Ælfric’s Colloquy, an early eleventh century schoolbook for teaching Latin, which among other things describes various occupations. It has this to say about a plowman:
Omni die debeo aratre [read arare] integrum agrum aut plus : ælce dæg ic sceal erian fulne æþer [read æcer] oþþe mare.
(Each day I must plow a full acre or more.)
(Source: Oxford English Dictionary, 3rd Edition)
Copyright 1997-2013, by David Wilton