What is the original reference to the "Sugar Plum Fairy"?

My dad was a die hard Beatles fan, and I grew up listening to their music.
At the beginning of one their songs, somebody whispers “Sugar Plum Fairy, Sugar Plum Fairy”
In another 60s song, (that my dad thought was cool because it sounded like jazz) a guy says “Sugar Plum Fairy came and hit the streets…”
Obviously, before the 60s, there was knowledge of something or someone called Sugar Plum Fairy.
And in “T’was The Night Before Christmas”, the children have visions of sugar plums.
So what exactly is a sugar plum?
And where did the Sugar Plum Fairy come from?