A blunt, somewhat old-fashioned insult for a stupid or slow-witted person. Not commonly used in modern casual speech; more likely encountered in older literature or as a deliberate, slightly comedic insult.
It's useless to say anything to such a blockhead.
Composed of 鈍 (どん, 'dull, slow') and 物 (ぶつ, 'thing, person'). The exact historical derivation is uncertain, but it is a straightforward compound meaning 'dull person'.