A reputedly repulsive creature from the Deneb system, best known because the Klingon officer Korax used its name to insult Captain Kirk aboard Deep Space Station K-7.
The Denebian slime devil is a creature associated with the Deneb system and regarded as thoroughly unpleasant. It is never seen in the original series; its fame rests entirely on its use as an insult.
In The Trouble with Tribbles, the Klingon officer Korax goads Scotty and Chekov in the bar of Deep Space Station K-7 by comparing Captain Kirk to a Denebian slime devil. Scotty lets the remark pass, and only throws a punch when Korax goes on to insult the Enterprise herself.
The scene is revisited from another angle in the Deep Space Nine episode Trials and Tribble-ations, when Sisko's officers find themselves in the same bar during the brawl. The phrase has since become one of the franchise's best-known pieces of abuse.