From Longman Dictionary of Contemporary Englishdeep downdeep downa) CERTAINLY/DEFINITELYif you know or feel something deep down, you secretly know or feel it even though you do not admit it He knew, deep down, that he would have to apologise. b) TRUEif someone is good, evil etc deep down, that is what they are really like even though they usually hide it Deep down, she is a caring person. → deep
Examples from the Corpus
deep down• Yeah, sometimes he can be really nice and polite but, I tell you, deep down he's an animal!• He pretends he doesn't care, but deep down I know he's very upset.• I regret my divorce, because deep down I'm a very old-fashioned woman.• I kept pushing the team, but deep down I think I knew we wouldn't win.• Deep down, I think she's really very ambitious.• I always believed deep down that things would get better. 