One way to hold plug-in is to picture the moment a plug meets a socket. Two things that were separate a second ago are now working as one, and neither had to be opened, cut, or rebuilt to make it happen. Pull it back out and both are exactly as they were. Every use of the word keeps that moment: a heater that joins the wall, a microphone that joins a laptop, a car that joins the house overnight, a small program that joins a bigger one.
From the word’s history
The supplied source marks plug-in as deverbal from the phrasal verb plug in: the action of pushing a plug into a socket was lifted whole and used as a name and as a describing word.
來源資料註明 plug-in 是從動詞片語 plug in 直接變來的（deverbal）。原本是「把插頭插進去」這個動作，整組被搬過來，一邊當名詞用，一邊當形容詞用。
Wiktionary entries supplied in this packet, etymology_text 'Deverbal from plug in.' (source_refs kaikki:3f6f69a190036ae4:1073808785 and :1073811960)
登入就可以看每一支怎麼連回核心畫面。