Green’s Dictionary of Slang

plumber n.1

[note the political ‘plumbers’ of the 1972–4 US Watergate Scandal, burglars rather than killers, but similarly deputed to ‘plug leaks’]

(US Und.) a hired killer.

Show quotations
[US]W.R. Burnett Little Men, Big World n.p.: The big boys just give a couple of the plumbers your address and that’s it. If the first set of plumbers don’t fix the leak, thy send another set.