What is "steam tug"

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.
   --Dryden.

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.