E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Fag.
One who does, and perseveres in doing. In public schools, it means a little boy who waits upon a bigger one. Probably a contracted form of factor, factotum; Latin, fac-re, to do.
1
Fag. Servant of Captain Absolute, who apes his master in all things. (Sheridan: The Rivals.)
2
Even the mendacious Mr. Fag assures us, though he never scruples to tell a lie at his masters command, yet it hurts his conscience to be found out.Sir Walter Scott.