dumb as a haddock en · ADJ
Meanings
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(not-comparable, qualifier:simile) Utterly mute; never speaking.
The brandy got into it and did his business for him, and Coo reeled off home, leaving his entertainer as dumb as a haddock on a Good Friday.
1850, Thomas Keightley, The Fairy Mythology, London: H.G. Bohn, page 534: