The seats on the bus that are oriented in the direction of travel and elevated above the seats in front. They are favoured by older ladies and do not feature a handlebar. So, in the event of a head-on collision, the passenger will be propelled in the air and towards the front of the bus, hence the name.
"Hey dude, wanna sit next to me in this 4-seat?"
"Nah man, I've got my eyes set on the granny cannon."