'backslider' originates from English, specifically the compound 'back' + 'slide' with the agentive suffix '-er'; 'back' meant 'at or to the rear' and 'slide' meant 'to slip or move smoothly'.

'backslider' developed from the verb 'backslide' (formed from Old English roots 'bæc' for back and 'slīdan' for slide) with the addition of '-er' to indicate a person who backslides; the noun has been used especially in religious English since early modern English periods.

Initially it described literally slipping or moving backward (from 'slide'), but over time it came to mean reverting morally or spiritually; the core idea of 'slipping back' persisted while the moral/religious sense became prominent.