If the literal meaning doesn't make sense, derive the meaning from the way it is used.

"To be honest" typically means "Here is an opinion that I'm embarrassed to share, and would rather lie about"

They're not lying about everything else, they're lying about that one thing, every other time.

e.g. "I tell people my favorite movie is 'The Godfather', but, to be honest, it's actually Ratatouille"

> Language is filled with those types of phrases, the one which bugs me once it was pointed out (even though I use it myself) is "to be honest...", which could carry the implication anything said without that qualifier may be dishonest.

Supernatural highlights this on S1E08, at 27:28. Dean was talking with someone and starts saying "the truth is" but the other person instantly cuts him off saying "you know who starts their sentences with 'the truth is'? Liars".