Slug

A slug is a type of shellless mollusk found on many planets.

Trill symbionts bare a certain resemblance to slugs, this comparison can be used in friendly conversation, however it can also be taken as an insult; In 2270 when Julian Bashir was describing Hortas to Jadzia Dax he described them as an "oversized slug made out of molten rock" which Dax took badly. Later when the two were at Quark's Kira Nerys stormed in and rebuked Bashir, once she departed Bashir noted that for a second he was worried she was going to slug him, again managing to insult Dax. (DS9 novel: Devil in the Sky)