What does "echoic origin" mean in the etymology explanation of bob?

"short, jerking motion," late 14c., probably connected to M.E. bobben "to strike, beat" (late 13c.), perhaps of echoic origin. Another early sense was "to make a fool of, cheat" (early 14c.). As a slang word for "shilling" it is attested from 1789, but the signification is unknown.