An ass which is only good when viewed in a container, such as jeans. Otherwise it is a big old jiggly ass.
"Man her ass looks real nice in them jeans"
"Dude, its probably just a container ass"
Container ass is used to describe an ass that only looks good because of the pants being worn. The definition works two ways in that the ass only looks good when in a container (pants) and the pants contain(h)er ass.
Damn, she has a hot ass! Yea, but take off the pants and I'll bet it's a container ass.