From Old English 'aecer' (field) — originally not a fixed size but what one yoke of oxen could plow in a day.
A unit of land area equal to 43,560 square feet (4,840 square yards), used principally in the US and UK.
From Old English æcer (a field, open land, a measured plot), from Proto-Germanic *akraz (field, pasture, open land), from PIE *h2eg-ro- (field, open country, land driven by ploughing), from *h2eg- (to drive). The sense of land driven by the plough — land worked by driving animals across it — connects agriculture and measurement. Latin ager (field), Greek agros (field, countryside), and Sanskrit
An acre was originally the area a single yoke of oxen could plow in one day — a strip one furlong (660 feet) long and one chain (66 feet) wide. Medieval fields were long and narrow because turning an ox team was slow and difficult. The shape of the acre literally reflects the stubbornness of oxen.