When a persons
(normally female)

Bum merges into their leg and upper thigh area,
and looses all ass definition
thus creating the impression of a legbum.
most common in old people or the generally unfit,
and also brought on by the wearing of high waisted or loose clothing, where the saggy bum is allowed to merge with the thighs.
this is most unnatractive.

An exclamation made when your friends or family are teasing you to a point where you can't handle it anymore and a hissy fit is in order.

Derived from a YouTube user's famed outburst following Britney's lackluster performance at the 2007 VMA's.