A common phrase used to describe a brunette, rather than a blonde, with low intelligence, practical experience, or common sense. It compares her intelligence to that of roadkill.
Wow, Jane just got a -14 on her math test... She's such a dumb deer.
Kandel is such a dumb deer! She keeps complaining that her pencil is backwards and that the eraser is where the point should be and the point is where the eraser should be!