Antonyms of must:
unessential, inessential, amenity, comfort, extra, extravagance, frill, indulgence, luxury, superfluity, surplus, surplusage, nonessential, nonnecessity.
Examples of usage:
  1. " You must drink it," said Jeff.
    "The Safety Curtain, and Other Stories", Ethel M. Dell.
  2. It must have been he, then!
    "What Will He Do With It, Book 7.", Edward Bulwer-Lytton.
  3. That must be him!
    "The Lights and Shadows of Real Life", T.S. Arthur Edition: 10 Language: English.