Meaning

noun someone who bolted from the Republican Party during the U.S. presidential election of 1884

noun a neutral or uncommitted person (especially in politics)

originThe noun is borrowed from Massachusett mugquomp, mummugquomp (“war leader”). Folk etymology reinterpreted it as referring to a person who sat on the fence with their mug (“face”) on one side and wump (“rump”) on the other, which influenced political cartoons during the 1884 United States presidential election. The verb is derived from the noun.

Etymology from Wiktionary, CC BY-SA 3.0