Webster's Revised Unabridged Dictionary · 1913

Mumm

Mumm , intransitive verb

[Dutch mimmen to mask, mom a mask; akin to German mumme disguise; prob. of imitative origin, and akin to English mum, mumble, in allusion to the indistinctness of speech occasioned by talking from behind a mask. Compare Mumble, Mummery.]

To sport or make diversion in a mask or disguise; to mask.
With mumming and with masking all around. — Spenser