E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Hair by Hair.
Hair by hair you will pull out the horses tail. Plutarch says that Sertorius, in order to teach his soldiers that perseverance and wit are better than brute force, had two horses brought before them, and set two men to pull out their tails. One of the men was a burly Herculs, who tugged and tugged, but all to no purpose; the other was a sharp, weasen-faced tailor, who plucked one hair at a time, amidst roars of laughter, and soon left the tail quite bare.