1. completely or to an extreme degree
shocked/rotten to the core
The 'core' of something is the centre or very essence of something or someone. If someone is described as being something 'to the core', it means 'in every way'. If you are 'kind to the core' for example, you are a very kind person in every possible way. If you are 'rotten to the core' you behave badly or are thoroughly awful. Describing something as 'to the core' can also mean 'to an extreme degree', usually used as a qualifier for intense emotions. This idiomatic expression is usually used for emphasis.