backside

After cycling for the whole day, my backside was very sore.

骑了一整天自行车后，我的屁股很疼。

On the backside of this building you will find a door.

在房子的后侧，你会看见一扇门。

Do not write anything on the backside of your answer book.

不要在答题本的背面写任何内容。