"one who or that which plugs" in any sense, 1867, agent noun from plug (v.). Specifically as "publicist" by 1913.
Entries linking to plugger
plug v.
"close tightly (a hole), fill or stop with or as with a plug," 1620s, from plug (n.) or from Dutch pluggen. OED dates the meaning "work energetically at" from c. 1865, and cites "Remembered on the river at Oxford" (and one wonders if a memory from Cambridge would pass). Sense of "popularize by repetition" is from 1906. Slang sense "put a bullet into" is recorded from 1870. Related: Plugged; plugging.