One way to hold this word: something that is pushed down inside or against something else, and passes the push on to whatever is in the way. A hand does the pushing in one group of uses, a machine in the other, but the picture stays the same — a part that travels down a tube or onto a hole, and moves what it finds there.
From plunge plus -er, so the word names the thing that does the plunging: the part that is pushed down.
由 plunge（猛地往下壓）加上 -er（做這件事的那個東西）組成，字面就是「負責往下壓的那一個」。
Wiktionary entry for plunger, supplied in this packet (kaikki:3f6f69a190036ae4:1176334752): 'From plunge + -er.'
Two branches
用手壓下去的那一段
這一組都是人用手壓，壓下去的目的各有不同：通開、擠出、壓到底。
noun · B1
A tool for clearing a blocked sink, bath or toilet: a rubber cup on the end of a stick, which you press down and pull up over the drain hole so that the water moves hard in both directions and loosens whatever is stuck.
通馬桶吸盤
機器裡來回滑動的零件
同樣是「壓下去、把東西推走」，只是動手的不再是人，而是幫浦、磁力或彈簧。
noun · C1
A part inside a machine that slides up and down, usually to push a liquid, a gas or another part along.
柱塞