late 14c., from O.Fr. moiste "damp," from V.L. *muscidus "moldy," also "wet," from L. mucidus "slimy, moldy, musty," from mucus "slime" (see mucus). Alternative etymology is from L. musteus "fresh, green, new," lit. "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.