Acre: a plot of land equal to 4,840 square yards. 

In Old English, an æcer was simply a “tilled field”.  Later, the word described the amount of land plowed by oxen in a single day.

This is an old word.  Variations show up in many ancient European languages.

In Latin, field is ager.  

In Greek, field is ἀγρός.  

In Old German, field is akraz.  

SInce these words are all similar, they are called cognates.  It is hard to say with certainty that our modern word “acre” comes from one language in particular.

An acre, superimposed on an American football field would roughly cover  the field from goal line to goal line.