What is a negative majority?

A negative majority is when neither major party in parliament (or other legislative agency that works based on a two-party dominant system) has an absolute majority of seats. This can also sometimes be called a "Hung Parliament".

https://en.wikipedia.org/wiki/List_of_United_Kingdom_general_elections

The German parliamentary election of 31 July 1932, held after the premature dissolution of the Reichstag, saw great gains by the Nazi Party, which for the first time became the largest party in parliament, though without winning a majority. With the Communists remaining strong, anti-republican parties together now had a majority in the Reichstag (called a "negative majority" by contemporary observers since the two parties would never have allied), making a (majority) government by any union of pro-republican parties impossible.

https://en.wikipedia.org/wiki/German_federal_election,_July_1932