One who makes dumb or silences; a person or thing that stupefies or confounds (archaic).
From a dialectal or archaic verb stem 'dummer' plus the agent suffix '-er'. This creates a noun for 'one who does the dumming.'
The '-er' suffix is so powerful in English—we add it to almost any verb to make 'the one who does it.' But 'dummerer' is so archaic it barely survives except in historical dictionaries!
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