It is possibly a reference to the song of the same title, "Where the Sun Don't Shine." It could also refer to the idiom of the same name.
There are some Squash, Peashooters, and Snow Peas with the Sunflowers. The zombies that are available are Zombie, Buckethead Zombie, and Football Zombie. Always place the zombies on the column right next to the red line. If there are no peashooting plants in a lane, place a regular Zombie there. If there is a Squash, put two but space them out or else the Squash might kill both zombies. If a lane has only Squashes, use four normal Zombies. If the defenses are below the minimum defense (see below), use a normal Zombie. Anything above, use a Buckethead Zombie. If there is a lane full of Snow Peas, a Football Zombie is recommended. Usually, the player should not have to use a Football Zombie at all and is also not recommend to overuse Football Zombies. Make sure to use normal Zombies to eat the brain if the lane is empty.