The term came from a Cornell University paper called "Unskilled and Unaware of It" by Justin Kruger and David Dunning. The paper describes that generally the more intelligent tend to underestimate their abilities because they know what they don't know. On the opposite side those the are ignorant tend to not know what they are talking about yet feel that they do.
In the paper an example is given of a man who smears lemon juice on his face because he thinks that it will keep cameras from seeing his face. After he robs a bank he is captured thanks to the bank camera footage. Even when shown the footage he could not comprehend how it didn't work. Hence the term lemon face.
Wife responds, "Lemon face."
Paris Hilton pretty much trying to do anything.
Any noob that insists competence
The act of scrunching your face to the point where it is so ugly that your body counter acts it with pain.
Me: Bruh, that dude got a lemon face! He ain't playin.
You: Oh, shit RUN!!
Me: DAMN! whats that smell?!?!
you: SHIT!!! LEMON FACE!!
2. The permanent facial expression of Ellen Degenerates, as if she'd "just smelled poo" or caught a whiff of sour milk from an old carton in the refrigerator.
"She started off with a few apple martinis, and ended up with a lemon face."
"She seemed to like the sex up until the climax, when she made a lemon face."