Definition from Wiktionary, the free dictionary
backside (plural backsides)
- The side of something that is opposite the front.
- The backside of the building faced an alley, and was covered in grime and scrawled graffiti.
- The buttocks.
- After riding the horse all day for the first time, I had painful blisters on my backside.
- See also Wikisaurus:buttocks
side of something