Having a stale, damp smell like old books or a closed room that hasn't had fresh air in a long time.
From 'must,' referring to mustiness or moldiness, derived from Old French 'moiste' (damp). The '-y' suffix converts the noun to an adjective describing something having that quality.
Every old library, attic, and antique shop has that distinctive musty smell—it's actually the scent of paper and leather slowly decomposing, plus mold spores, which somehow became romantic and nostalgic instead of just gross!
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