must ‧ y /ˈmʌsti/ BrE AmE adjective
[ Date: 1500-1600 ; Origin: Perhaps from moisty 'wet' (15-19 centuries) , from moist ; influenced by ⇨ ↑ must 2 (2) ]
a musty room, house, or object has an unpleasant smell, because it is old and has not had any fresh air for a long time:
the musty smell of old books