Definition
A directive for someone to calm down or modify their attitude, and to refrain from speaking about an issue in which they have limited knowledge or experience.
Origins
From have a seat (and take a seat).
How People Actually Use It
When somebody says something so stupid it's beyond repair. The act of being basic in the presences of a G.
"Trina: Beyoncé 's hair is all hers no weave Alicia: go have several seats"
Source: Urban Dictionary