marplot
/ˈmɑːplɒt/noun
A person who or (occasionally) a thing which spoils a plot or hinders the success of any undertaking.
- In early use allusively as a personification..
adjective
That spoils or defeats a plot or hinders an undertaking. Now rare.
Origin
Early 18th century; earliest use found in Susanna Centilivre (bap. 1664, d. 1723), playwright and actress. From mar- + plot.