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.

When some little bitchboy is acting like a little tiny pee pee poo poo baby and messing up the group plans.