See have a cow. "Don't have a cow" is a trademark expression for the TV series' favorite son, Bart Simpson. Bart is the typical problem child who is always testing the authority figures in his life. In contrast is the good child, Lisa Simpson, who is never a challenge to her elders -- and often fades into the woodwork because of it.
Lisa started to freak out when Bart took the bottle of rum from the cabinet, poured one cup out, then filled the bottle back up with water. Bart socked Lisa in the arm and said, "Don't have a cow!"