1
general::
Phrase(s): rotten to the core
Fig. really bad; corrupt. • That lousy punk is rotten to the core. • The entire administration is rotten to the core.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Thoroughly bad, as in It seems that this police unit is rotten to the core, involved in numerous extortion schemes. The noun core here denotes the central part or heart of anything or anyone. The idiom was first recorded in 1804.
American Heritage Idioms