Every human being has seen him or herself in the mirror, and come to a conclusion about what angle, amount of lighting, and expression make for what they consider to be their "best face". Girls are more prone to this activity than guys, though gender does not preclude the act from taking place eventually.
On the other hand, there are moments in our lives, specifically during some kind of social activity, where we are paying little to no attention to the angle, lighting, and expression upon our faces. Sometimes, this can result in a starkly different - and often alarming - physical appearance of our faces, compared to the normally controlled "look" we present to others.
This appearance is called Primal Face. It is the most accurate and no-holds-barred visual representation of what we look like. Many people are not consciously aware of the existence of Primal Face, but they certainly do see it, when it happens. The most gorgeous, seemingly perfect guy or girl can seemingly degenerate into a hideous, fugly frumpbomb when in the midst of their own sudden and momentary Primal Face.
Ingus: What?? What happened?
Bob: Okay... Okay....hold on a sec...lemme lower my heart rate...
Ingus: Geez man, what is it?
Bob: I was talking to Katie, and you know she's super hot, right? Well, I was telling her a story of how I got into acting, and her eyes glazed over, like not really listening, you know?
Ingus: Yeah, I know that look..
Bob: Okay, well... I looked away for a sec while I was talking, and when I looked back, Katie was gone...in her place was some kind of wretched booby-beast! All these wrinkles and indentations on her face showed up outta nowhere, and was giving me the fish-eye! It was scary, dude..
Ingus: Oh crapola... Listen closely man, listen: what you saw was her Primal Face.. The most uglifying frame of visual representation.. It happens to everyone.
Bob: Really? Wow...phew, that's a relief. I thought I'd been blinded by lust and had woken up.