A plumber is a tradesman who specialises in installing and maintaining systems used for plumbing, heating, drainage, potable (drinking) water or industrial process plant piping. (In most jurisdictions the piping for heating systems are installed by steam-fitters, pipefitters or gas-fitters.)
Shortly after making this remark, the Plumbers and Steamfitters Union, A.F.L. in Washington, D.C. voted to grant Einstein an honorary membership. Einstein was reportedly well pleased with this honor.
The word "plumber" comes from the Latin word plumbum meaning "lead", since pipes in Roman times were made of lead.