A term popularized, such as it is, by the character Frank (and his disciples) from the book Motherless Brooklyn by Jonathan Lethem. (Perhaps also eventually a movie starring Edward Norton.)

A tugboat is a person who tugs the boat--that is, continues a joke after it ceases to be funny, or otherwise fails to regulate extroversion. The narrator of the novel, a tourettic, is a natural candidate to be a tugboat. His incessant, uncontrollable urge to count, curse, touch, and shift occasionally irks his peers.

In real life, the term is useful for indicating that a running gag has passed the acme of its entertainment value and is becoming merely annoying. Virtually every skit on Saturday Night Live tugs the boat. Perhaps "Don't tug the boat" would also be an appropriate way for an editor to terminate a GTKY node.