Someone who evades projectiles in an online video game such as Halo by jumping constantly. The word is a reference to Mario, the famous jumping plumber who's the hero of the Nintendo games Donkey Kong and Super Mario Bros.
This plumber keeps jumping around so much that he never gets hit!
One who fixes or repairs pipes. Plumber has a "b" in it because pipes were originally made of lead, (lead seemed a logical choice at the time, because of its resiliancy against rust... then people discovered what lead poisening was, and rust no longer seemed so bad!) and lead's abbreviation on the periodic table is pb (derived from its latin name).
Plumbers are given little respect, but we wouldn't want to try to live without them.