Why on earth is it sometimes called a 'backside'?

@zugedanit (1958)
Philippines
February 12, 2007 4:12pm CST
That part of the body which is officially titled the gluteus maximus also goes by other names such as behind, derriere, posterior, rear and quite a few colorful names as well. However, why on earth is it sometimes called a 'backside'? The bottom of our back is not at the side. Just one of those things that you ponder.
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