The default response used by someone who is confronted with a truth about themselves that they would rather not face.

This phrase is commonly used by addicts, those with low self esteem and just about anyone else whose biggest fear is that someone else will notice their problem and offer to help.

Using this phrase successfully pushes away anyone wanting to help, and the user ironically ensures that indeed, nobody will ever know them.

In another ironic twist, if someone you know well says this phrase to you, then it means either:

1) they've been lying to you about who they really are (in which case it's their own fault that you don't know them), or

2) they have suddenly and fundamentally changed in the few seconds prior to telling you that you don't know them (in which case they also don't know themselves).

However, the case is usually that they're unobservant, self-absorbed, don't like that you were so easily able to identify their issues and resent the fact that they are unable to pay attention to anything long enough to 'know' it. They assume that everyone else is like this too, which is why they seem so shocked when uttering this phrase.
"I think you've been ignoring your family and you should spend more time with them."
"You don't even know me!"

"I think your drinking is negatively affecting your performance at work."
"You don't even know me!"

"I think you should take a shower because you stink."
"You don't even know me!"
by Jerk Nowitzki April 10, 2008