"Mertz!" was a expletive uttered at the Battle of Donatu V by a besieged Star-Colonel, in response to a Klingon demand for surrender. It created such confusion among the Klingon translators that reinforcements had time to arrive and save the day for the Federation.
It was first mentioned in David Gerrold's novel,Star Trek: The Galactic Whirlpool."
Mertz was probably not an expletive at all, but rather a nonsense word. As such, it has no analog in American English.