pain

Pain comes from Middle English peyne, from Old French and Anglo-Norman peine, from Latin poena ("punishment, pain"), which traces back to Ancient Greek poinḗ, meaning bloodmoney or a fine paid as penalty, ultimately from a Proto-Indo-European root meaning "payment."

What began as a word for a penalty or payment owed for wrongdoing narrowed over time into the felt experience of suffering itself — the punishment became the hurt. In English, it largely displaced the native Old English word sār, which survives today as "sore."

It's a doublet of "peine" (as in the phrase "peine forte et dure"), and shares a root with the verb "pine," as well as German Pein, Dutch pijn, and Danish and Norwegian pine — all carrying the same old idea of suffering.