One who esteems or holds something in low regard; a person who underestimates the value or worth of something.
From de- (reverse or reduce) + -seem- (from Latin 'seems' or 'appear') + -er (one who does). This is an archaic or rare word from older English, appearing occasionally in historical texts to describe someone who diminishes the value of things.
This is a wonderfully obscure word—you almost never see it used today, but it shows how English once had words for very specific types of people ('a desemer' = someone who undervalues things), just like we had 'scold' for a bad-tempered person.
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