molunō

This is the New Testament's most concrete, down-to-earth word for defilement. Its root picture is smearing something with mud, filth, or blood, and it always carries a note of disgrace, never a neutral or ceremonial one. The everyday examples behind the word are a tunic ruined by dirt and a pig wallowing in mud, so when the New Testament writers reach for it, they mean to make a stain feel visible and shameful.

All three New Testament uses are figurative, and they fall into two related pictures. In one, an inner faculty is stained: a person's conscience is left soiled by acting against their own conviction. In the other, a person's outward life is stained by drifting into idolatry and compromise with pagan worship, pictured as dirty clothes or as defiling oneself. In every case what is polluted is the moral and spiritual life, portrayed with the strongest image Greek had for something disgracefully filthy.

The writers could have chosen other words for defilement, but did not. They avoided μιαίνω, the standard term for religious or ritual pollution, and κοινόω, the word for making something ceremonially "common." By choosing this word instead, Paul and John describe a real moral stain without invoking the machinery of ritual purity: the damage is genuine and visible in a person's life, not a matter of ceremonial status.