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.
late 14c., mussillage, "viscous substance found in vegetable material," from Old French mucilage (14c.) and directly from Late Latin mucilago "musty or moldy juice" (4c.), from Latin mucere "be musty or moldy," from mucus "mucus" (see mucus). Meaning "adhesive gum" is attested by 1859.
"viscid fluid secreted by the mucous membranes of animals," 1660s (replacing Middle English mucilage), from Latin mucus "slime, mold, mucus of the nose, snot," from PIE root *meug- "slippery, slimy," with derivatives referring to wet or slimy substances or conditions (source also of Latin emungere "to sneeze out, blow one's nose," mucere "be moldy or musty," Greek myssesthai "to blow the nose," myxa "mucus;" Sanskrit muncati "he releases"). Old English had horh, which may be imitative.
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.).