The Collaborative International Dictionary of English v.0.48:

Muggins \Mug"gins\, n. [Etym. unknown.]
   1. A game of dominoes in which the object is to make the sum
      of the two ends of the line some multiple of five.
      [Webster 1913 Suppl.]

   2. A game at cards which depends upon building in suits or
      matching exposed cards, the object being to get rid of
      one's cards.
      [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Muggins \Mug"gins\, v. t.
   In certain games, to score against, or take an advantage over
   (an opponent), as for an error, announcing the act by saying
   "muggins."
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

muggins
    n 1: a person who lacks good judgment [syn: fool, sap,
         saphead, muggins, tomfool]