An illegal play, shooting out of turn, ball tampering, rampant stupidity or intentional game prolongation are all grounds for a hue and cry of "Muggins!" Generally this results in a Doubling Round for all the other Players. The term originated in Pinochle.
Muggins is taking points your opponent misses. Muggins points come from one of two sources. If your opponent misses points during the play of the cards, Muggins may be called after a pause to make sure the opponent is truly missing the points. The other situation is where your opponent fails to count and/or take all the points in the hand. Once he has under pegged his hand, Muggins may be called for the difference in what was pegged and the actual count in the hand. Muggins may not be taken in the case of an opponent failing to take his two points when a Jack is the starter card, those points are lost once the dealer plays a card. Muggins is not applicable in the case where an opponent over pegs his hand, see Over Pegging - Hauling Lumber