The Collaborative International Dictionary of English v.0.48:
Musty \Mus"ty\, a. [Compar. Mustier; superl. Mustiest.]
[From L. mustum must; or perh. fr. E. moist. Cf. Must, n.,
1. Having the rank, pungent, offensive odor and taste which
substances of organic origin acquire during warm, moist
weather; foul or sour and fetid; moldy; as, musty corn;
musty books. --Harvey.
2. Spoiled by age; rank; stale.
The proverb is somewhat musty. --Shak.
3. Dull; heavy; spiritless. "That he may not grow musty and
unfit for conversation." --Addison.