I wondered, 'Why did that word pop into my head?'
And then I realised my subconscious was at work, thinking of the etymological roots of these two words.
But what does acting with aplomb have to do with plumbing? I thought they must have some connection to the old Latin word for lead, because a plumb line is a line with a lead weight on the bottom. Gravity pulls such a line straight and shows where the vertical is. Here's the origin of plumb.
Coincidentally, at dinner tonight we were discussing whether ancient Romans died young from lead poisoning because they used lead pipes to transport water, and one family member maintained that lead is not soluble in water. Here's some information about the role of lead poisoning in the fall of the Roman Empire.
The Online Etymology Dictionary says this about the word plumber:
c.1100, "a worker in any sort of lead," from O.Fr. plummier (Fr. plombier), from L. plumbarius "worker in lead," properly an adj., "pertaining to lead," from plumbum "lead" (see plumb). Meaning shifted 19c. to "workman who installs pipes and fittings" as lead water pipes became the principal concern of the trade. In U.S. Nixon administration (1969-74), the name of a special unit for investigation of "leaks" of government secrets. Plumbing "water pipes" is first recorded 1884.And about aplomb:
"assurance, confidence," 1828, from Fr. aplomb (16c.), lit. "perpendicularity," from phrase à plomb "poised upright, balanced," lit. "on the plumb line," from L. plumbum "(the metal) lead" (see plumb), of which the weight at the end of the line was made.