You can think of a retainer as the one whose whole job is to keep something from leaving. Something wants to drift away — teeth that want to move back, a part that wants to slide out, a lawyer who might take another client, a room that could be rented to someone else — and the retainer is what holds it where it is. Ask what is being held, and you usually have the meaning.
From the word’s history
Wiktionary records retainer as retain + -er — the -er that names the person or thing that does something. So the word is literally the one that retains, and every modern use is a different answer to the question of what is being retained.
來源字典把 retainer 拆成 retain（留住）加 -er（做這件事的人或東西），所以字面就是「會留住東西的那一個」。今天的每個意思，都只是對「留住什麼」給了不同的答案。
Wiktionary, English entry 'retainer' (packet source_ref kaikki:3f6f69a190036ae4:966000018:18594): 'From retain + -er.'
登入就可以看每一支怎麼連回核心畫面。