Similar to 'traitor,' it is a word chanted by those supported by, or who won over the emperor or Imperial Court in the era, to conspire political elimination of the political (or at times, military) adversary.
「逆賊」などと同じで、各々の時代において、天皇、朝廷の支持を得た側、味方に取り込んだ側が、政治的（時には軍事的）対立者の政治的抹殺を謀って唱えた言葉。