The Collaborative International Dictionary of English v.0.48:

Mobcap \Mob"cap`\, n. [D. mop-muts; OD. mop a woman's coif + D.
   muts cap.]
   A plain cap or headdress for women or girls; especially, one
   tying under the chin by a very broad band, generally of the
   same material as the cap itself. --Thackeray.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mobcap
    n 1: large high frilly cap with a full crown; formerly worn
         indoors by women