musty (adj.)

late 14c., "slightly wet; well-irrigated, characterized by moistness," from Old French moiste "damp, wet, soaked" (13c., Modern French moite), which is of uncertain origin. Perhaps from Vulgar Latin *muscidus "moldy," also "wet," from Latin mucidus "slimy, moldy, musty," from mucus "slime" (see mucus). Alternative etymology [Diez] is from Latin musteus "fresh, green, new," literally "like new wine," from musteum "new wine" (see must (n.1)). If this wasn't the source, it influenced the form of the other word in Old French. Related: Moistly; moistness (mid-14c.).

"mold, moldiness," c. 1600, perhaps a back-formation of musty (q.v.).