(covers information from several alternate timelines)
A plumber was an occupation that specialized in plumbing assembly and repair.
In 2263, a miffed Leonard McCoy, upon learning that Spock suggested he accompany him onto a swarm ship to disorient the others, he snapped "Next time you have a piece of pipe stuck in your transverse... call a plumber." (Star Trek Beyond)
Montgomery Scott regarded his 2267 temporary repair to the PXK pergium reactor as being "a plumber's nightmare," that is, he had used a mix-match of odds-and-ends to keep in running, but ensured James T. Kirk that it would "hold for a bit." (TOS: "The Devil in the Dark")