-I know, why would someone lie about that?
So this statement, in effect, when used, devalues any potential credibility one once had as an honest and truthful person when they are not using the statement.
It is synonymous with: to tell you the truth, honestly, actually, as a matter of fact, in fact , truthfully
I'm not gonna lie, this taste terrible.
I'm not gonna lie, you are really good at what you do.