From Longman Dictionary of Contemporary Englishat heartat heartCHARACTER/PERSONALITYif you are a particular kind of person at heart, that is the kind of person that you really are even though you may appear or behave differently He may be a working class boy at heart, but his lifestyle has been transformed. Let’s face it, we’re all romantics at heart. → have somebody’s (best) interests at heart at interest1(5), → young at heart at young1(5) → heart
Examples from the Corpusat heart• Paul was an easy-going fellow at heart who wanted only to enjoy himself.• I guess I'm just a kid at heart.• She's a traveller at heart. You'll never get her to settle down.