you're regular street clothes that you wear when out of uniform. this includes any uniforms: work, band, military, school, so on.
also refers to costumes or any other required dress you wouldn't be seen in on a day to day basis.

A stupid person; it refers to the lack of surface area on an individual's brain. The general thought is that the more surface area (wrinkles, creases, etc.) a brain has, the smarter the person is. Conversely, a person with a smooth brain (no wrinkles) has less surface area and would therefore be stupid.

"Civilian" a term used by members of the military.

Civillians, often used by military personelle.

at schools with uniforms days where you can wear watever you want. (usually once a month)