marplot

[ mahr-plot ]

/ ˈmɑrˌplɒt /

a person who mars or defeats a plot, design, or project by meddling.

Origin of marplot

First recorded in 1700–10; mar + plot

Our only marplot has been laid by the heels at the very nick of time.

Cripps, the Carrier |R. D. (Richard Doddridge) Blackmore
Jaqueline had followed her sister, though she knew Varina was generally the marplot in their play.

A Little Girl in Old Washington |Amanda M. Douglas
“The marplot, madam, has the instinct of his nature,” returned Gondremark.

The Works of Robert Louis Stevenson - Swanston Edition Vol. 7 (of 25) |Robert Louis Stevenson
Marplot, an officious bungler, who spoils everything he interferes with.

The Slang Dictionary |John Camden Hotten

Holding fast to these beliefs, Cimarron Bill rode forth full of an unconscious willingness to play the marplot.

The Sunset Trail |Alfred Henry Lewis