slag [M16th] Slag is from Middle Low German slagge, perhaps from slagen ‘strike’, with reference to fragments formed by hammering. From the sense of refuse developed a slang use meaning ‘worthless person’ in the late 18th century, which only developed the modern dominant sense ‘promiscuous woman’ in the mid 20th century. The verb sense ‘criticize’ as in slagged off, dates from the 1950s.