I have heard this term from a Nelly "apple bottom" model search. He was looking for a girl with a bottom that is fuller figured but athletic. He has a brand of apple bottom clothing that is made for the "fuller bottomed" girl.
applebutt, is a firm apple shaped butt, no jiggle... there is apple, pear, athletic.... then of course you have the obese and heavy butts... I have a book called the Butt Book that has all of them in it... Rhonda