acre (n.) Look up acre at Dictionary.com
Old English æcer "tilled field, open land," from Proto-Germanic *akraz "field, pasture" (source also of Old Norse akr, Old Saxon akkar, Old Frisian ekker, Middle Dutch acker, Dutch akker, Old High German achar, German acker, Gothic akrs), from PIE *agro- "field" (source also of Latin ager "a field, land," Greek agros "a field, a farm; the country," Sanskrit ajras "plain, open country").

"[O]riginally 'open country, untenanted land, forest'; ... then, with advance in the agricultural state, pasture land, tilled land, an enclosed or defined piece of land" [OED]. In English at first without reference to dimension; in late Old English the amount of land a yoke of oxen could plow in a day, afterward defined by statute 13c. and later as a piece 40 poles by 4, or an equivalent shape [OED cites 5 Edw. I, 31 Edw. III, 24 Hen. VIII]. The older sense is retained in God's acre "churchyard." Adopted early in Old French and Medieval Latin, hence the Modern English spelling, which by normal development would be *aker (compare baker from Old English bæcere).