Picture someone standing in a corridor full of doors, watching to see which one swings open. The moment one does, they walk through it. What makes them an opportunist is not their speed but the question they ask. They do not ask whether it is right, or whether they planned for it. They ask only whether it is open right now. Both uses of the word grow from that one habit: the person who changes their promises to fit the moment, and the person who takes the bike because it happened to be unlocked.
Recorded as a borrowing of French opportuniste, from opportune plus the ending -ist: the person whose speciality is the right moment.
依據來源，這個字借自法文 opportuniste，由 opportune（時機剛好）加上 -ist（某種人）構成。字面就是「專做『時機剛好』這件事的人」，正好對上今天兩個用法裡共有的畫面。
Wiktionary entry supplied in this packet (source_ref kaikki:3f6f69a190036ae4:823902389:17017): 'From French opportuniste, equivalent to opportune + -ist.'
Two branches
抓機會替自己爭好處的人
這一支最貼近核心畫面：門一開就進去，不問該不該。差別只在於，這裡走進去換到的是權力、金錢或面子。
noun · C1
Someone who takes any chance that appears to get power, money, or an advantage for themselves, even when that means dropping what they said they believed or treating other people unfairly. The word usually shows that the speaker does not approve.
投機者、機會主義者
因為剛好有機會才犯案的人
同一個畫面換到治安的場景：這次「打開的門」是真的沒鎖的門、沒關的窗、桌上的手機。
noun · C1
Someone who steals or breaks the law because a chance suddenly appeared — an open window, an unlocked bike — and not because they planned it beforehand.
臨時起意的犯案者