The Collaborative International Dictionary of English v.0.48:

Mumm \Mumm\, v. i. [imp. & p. p. Mummed; p. pr. & vb. n.
   Mumming.] [D. mimmen to mask, mom a mask; akin to G. mumme
   disguise; prob. of imitative origin, and akin to E. mum,
   mumble, in allusion to the indistinctness of speech
   occasioned by talking from behind a mask. Cf. Mumble,
   Mummery.]
   To sport or make diversion in a mask or disguise; to mask.
   [1913 Webster]

         With mumming and with masking all around. --Spenser.
   [1913 Webster]