Deep down refers to something that is at the core of a person's beliefs or character. It can also indicate that a person holds a particular opinion or feeling but does not express it openly. There are several synonyms for this phrase, including fundamentally, at heart, essentially, fundamentally, at the core, intrinsically, inherently, and basically. Each of these words describes an aspect of a person's innermost thoughts or beliefs. Ultimately, they represent the truest, most sincere aspects of a person's personality. When we speak of deep-down qualities, we are referring to those things that make a person unique and authentic.