sin

Sin comes from Old English synn, from Proto-West Germanic *sunnju, tracing back to a Proto-Germanic root meaning 'truth, excuse' as well as a separate root meaning 'sin.' Further back lies the Proto-Indo-European *h₁sónts, meaning 'being, true' — used to describe someone truly, verifiably guilty when accused of something.

The root idea was about being proven true against a charge — a verdict of 'truly guilty' — which narrowed over time into the general concept of moral wrongdoing itself.

It's a doublet of suttee, and cognate with Gothic sunja ('truth') and Latin sons ('guilty, criminal'), showing the same tangle of truth and guilt across languages. It's also related to English sooth, from Old English sōþ ('true').