A person who returns to bad behavior or worse conditions after improvement; someone who loses faith or abandons previously held beliefs.
From 'backslide' + '-er' (one who does), originating in religious communities to label those who abandoned faith. It evolved into general use for anyone who regresses.
The word 'backslider' carries heavy moral judgment—it's not neutral like 'relapsed' but implies willful betrayal, which is why religious preachers used it to shame people back into line.
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