E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
An arrow. The feathers under the beak of a hawk were called barb feathers (beard feathers). The point of an arrow has two iron feathers, which stick out so as to hinder the extraction of the arrow. (Latin, barba, a beard.)
N.B.The barb is not the feather on the upper part of the shaft, but the hooked iron point or head.