Kiersten went into Casey's to get cigarettes big gulps doughnuts and condoms and when she came back she found Chad fucking Mackenziein the Backseat of her car!
Is a reference to a person who frequently asks "what?". The term originates from people in the back of the car always misinterpreting the message of the people in the front seats. The term is properly used as a noun.
Backseat is when something is horrible and sucky. This makes perfect sense because no one likes to sit in the backseat, and sometimes nasty things happen in backseats. This will catch on in the near future.