Musty

google

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early 16th century: perhaps an alteration of moisty ‘moist’, influenced by must2.

wiktionary

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must +‎  -y. 

etymonline

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musty (adj.)

1520s, "moldy, sour," perhaps a variant of moisty "moist, damp" (see moist), but musty, of bread, "containing must" is attested mid-15c., and from 15c.-19c. must also was a variant of musk. Related: Mustiness.