Definition from Wiktionary, the free dictionary
Folk etymology reinterpreted it as referring to a person who sat on the fence with mug ("face") one one side and wump ("rump") on the other, which influenced political cartoons during the 1884 U.S. presidential election.
mugwump (plural mugwumps)
- (US) An independent neutral politician, especially in reference to the 1884 U.S. presidential election.
- (colloquial) An aloof or inconsequential person.
1935, George Goodchild, chapter 1, in Death on the Centre Court:
- “Anthea hasn't a notion in her head but to vamp a lot of silly mugwumps. She's set her heart on that tennis bloke […] whom the papers are making such a fuss about.”