The Collaborative International Dictionary of English v.0.48:
Tug \Tug\, n.
1. A pull with the utmost effort, as in the athletic contest
called tug of war; a supreme effort.
At the tug he falls,
Vast ruins come along, rent from the smoking walls.
2. A sort of vehicle, used for conveying timber and heavy
articles. [Prov. Eng.] --Halliwell.
3. (Naut.) A small, powerful steamboat used to tow vessels;
-- called also steam tug, tugboat, and towboat.
4. A trace, or drawing strap, of a harness.
5. (Mining.) An iron hook of a hoisting tub, to which a
tackle is affixed.
Tug iron, an iron hook or button to which a tug or trace
may be attached, as on the shaft of a wagon.