must + -y.
musty (comparative mustier, superlative mustiest)
- Having a stale odor.
1913, Joseph C. Lincoln, chapter 12, in Mr. Pratt's Patients:
- So, after a spell, he decided to make the best of it and shoved us into the front parlor. […] It looked like a tomb and smelt pretty nigh as musty and dead-and-gone.