1520s, "to chew noisily, crunch;" 1570s (of horses) "to bite repeatedly and impatiently," probably echoic; OED suggests a connection with jam (v.). Earlier also cham, chamb, etc. (late 14c.). To champ on (or at) the bit, as an eager horse will, is attested in the figurative sense by 1640s. Related: Champed; champing. As a noun, "act of biting repeatedly, action of champing," from c. 1600.
"a field," c. 1300, from Old French champ, from Latin campus "flat land, field" (see campus).