means A plumbing tool with a rubber cup on a handle, used to clear clogs in drains and toilets by creating suction and pressure.

from From the verb "plunge," which came through Old French "plongier" from a Late Latin root "plumbicare," meaning to heave the lead — a sounding weight (Latin "plumbum," lead) dropped to measure water depth. So the word is, fittingly, soaked in the language of diving down into water. The "-er" ending simply names the thing that does the plunging — the household tool sense being a comparatively modern application.