If you know something deep down or deep down inside, you know that it is true, but you are not always conscious of it or willing to admit it to yourself.
We knew deep down that we could do it.
Deep down, we had always detested each other.
See full dictionary entry for deep
deep down in British English
informal
in reality, esp as opposed to appearance
she is a very kind person deep down
See full dictionary entry for deep
Examples of 'deep down' in a sentence
deep down
We know deep down there is more.
Christianity Today (2000)
We disagree deep down in our blood and bones.
The Sun (2015)
Deep down we all know how to eat.
Times, Sunday Times (2014)
Deep down, you know you are ready to take on more at work.
The Sun (2008)
There was an element of me showing off, because deep down I suffer from a huge inferiority complex.
Times, Sunday Times (2009)
I watch all these programmes on TV and deep down I wonder,'What have they got that I haven't?