A derogatory term used to describe a person that is unaware of their ignorance on a certain topic despite insistence that they know what they are doing.
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.
"I'm not going to call a plumber honey because I can do it."
Wife responds, "Lemon face."