E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
To take the cattle of another to graze at a certain sum. The feeding of these beasts is called agistment. The words are from the Norman agiser (to be levant and couchant, rise up and lie down), because, says Coke, beasts are levant and couchant whilst they are on the land.