rotten to the core

:  having rotted :  putrid

:  morally corrupt

:  extremely unpleasant or inferior

:  to an extreme degree

:  a central and often foundational part usually distinct from the enveloping part by a difference in nature : such as

:  the usually inedible central part of some fruits (such as a pineapple)

:  the papery or leathery carpels composing the ripened ovary in a pome fruit (such as an apple)

:  to remove a core (see ¹core 1a) from

:  a group of people