must must /ˈmʌst/

adjective

1. Highly recommended  ( must )
   highly recommended
    A book that is must reading
   a book that is must reading
   

noun

1. The quality of smelling or tasting old or stale or mouldy  ( moldiness [ N. American] , must , mustiness , mouldiness [Canadian, British] )
   the quality of smelling or tasting old or stale or mouldy
   
2. Grape juice before or during fermentation  ( must )
   grape juice before or during fermentation
    Before fermentation, the grape juice, known as must, was stored in large vats.
   Before fermentation, the grape juice, known as must, was stored in large vats.
   
   
3. A necessary or essential thing  ( must )
   a necessary or essential thing
    Seat belts are an absolute must
   seat belts are an absolute must