Butt
Buttocks, Bottom
noun
The お is just there to be nice to your butt. This word simply means butt (or any other word for it, like buttocks or bottom).
The reading is the same as the one you learned with the kanji.
高いところに行くと、お尻がかゆくなるんです。
When I go to high places, my butt itches.
赤ちゃんのお尻ほど可愛いものはこの世に無い。
There's nothing in the world that's cuter than a baby's butt.
ほとんどの日本人が誕生日にお尻を叩くなんて聞いた事がないと思うよ。
I bet most Japanese people have never heard of a birthday spanking.