Simply.....Plumbers.
The word speaks for itself.
Most other trades have names that are / were / remain derogatory to their trades and ability to work and perform as tradesmen ( or tradespersons to be politically correct )
Unfortunately..in the general consensus, plumbers are highly skilled and not deserved of rudimentary, demeaning or belittling comments about us and our trade.
If this was the case, a universal term would be commonly used and would had been used for many hundreds, if not thousands of years.
We might be as proud of showing the world our bum crack as any aspiring super model showing her bikini bridge.
You don't hear of a carpenters, electrician or painters crack...do you?
It comes down to the fact that we are plumbers - and every other trade is jealous.
They are all 'closet plumbers'