Achuron (ἄχυρον) refers to chaff — the dry husks and straw separated from grain during winnowing. It appears in Matthew 3:12 and Luke 3:17 in John the Baptist's proclamation that the Coming One will "burn up the chaff with unquenchable fire."
John the Baptist's image of chaff (achuron) and the winnowing fork is one of Scripture's most vivid judgment metaphors. Threshing is not destruction of grain but separation — the wheat is gathered while the chaff is burned. The same pneuma (wind/spirit) that blows the chaff away is the Spirit who empowers the righteous. Proximity to the grain is not the same as being grain. The Pharisees who came to John looked like religious wheat but were chaff in God's sight. Judgment is separation by truth.