A harsh, discordant sound or cry, particularly an unpleasant noise from a musical instrument.
From French couac, likely imitative in origin, mimicking the harsh sound itself—similar to how 'croak' and 'quack' sound like what they describe.
Couac is one of those words where the sound of the word actually matches what it means—it's an onomatopoeia borrowed from French, and musicians use it to describe that embarrassing squeak or honk that comes out when you hit a wrong note.
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