" rotten to the core "

[Idiom]  | "rotten to the core" 

1: very bad or dishonest

[Idiom]  | "to the core" 

1: in a very complete or extreme way —used for emphasis

[Adjective]  | "rot*ten" | \ ˈrä-tᵊn \

1: having rotted : putrid

2: not morally good : not kind, loyal, honest, etc.

3: extremely unpleasant or inferior

Origin: 13th century ;

 Middle English roten, from Old Norse rotinn; akin to Old English rotian to rot;