Fromim- +‎mask.

immask (third-person singular simple presentimmasks,present participleimmasking,simple past and past participleimmasked)

1. (obsolete) To cover, as with amask; todisguise orconceal.
   • c.1597 (date written),William Shakespeare, “The First Part of Henry the Fourth, […]”, inMr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, andEd[ward] Blount, published1623,→OCLC,[Act I, scene ii]:

     immaske our noted outward garments

   • 1806, Andrew Becket,Socrates (poem):

     The thick, dark mists of hell
     Which envelop gaunt envy and hate
     Thusimmask'd, and in seeming security

• “immask”, inWebster’s Revised Unabridged Dictionary, Springfield, Mass.:G. & C. Merriam,1913,→OCLC.

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