late 14c., "a dog;" late 15c., "noisy fellow;" agent noun from bark (v.). Specific sense of "loud assistant in an auction, store, or show" is from 1690s.
Entries linking to barker
bark n.1
"tree skin, hard covering of plants," c. 1300, from a Scandinavian source akin to Old Norse börkr "bark," from Proto-Germanic *barkuz, which probably is related to birch and Low German borke. The native word was rind.