If it pushes.....

[Keygrip]

If the thing PUSHES aircraft back from the gate....why is it called a "tug"?

[ATCO Two]

Because the "thing" also tows (tugs) aircraft around the airport, and to and from the stands. A tug is sometimes also called a pushback car/truck in some parts of the world, where English is not the first language.

[LHmain]

Ever seen a tug put a ship in dock?

[woderick]

Towing (tug) came first, pushback much later idea, same equipment used, name sticks.