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-ere, to do.
?Even the mendacious Mr. Fag assures us, though he never scruples to tell a lie at his master's command, yet it hurts his conscience to be found out.? ?Sir Walter Scott.