I am re-reading Ringworld now, and I came across this term; does anyone know where I can find the origin in Niven's canon of work?
A reaction drive is a weapon effective in proportion to it's efficiency.
The in-universe origin can be traced back to the first contact between Humans and Kzin where
the drive on the Angel's Pencil (a laser propelled slowboat) is used to destroy a Kzinti warship despite the Kzinti telepath's assessment that the Humans have "no weapons".
(told in "The Warriors") and the subsequent Kzinti attempts to invade the solar system.
Just now I'm not positive about where the phrase first appeared in text, but the Ringworld mention has to be close to the first.