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1.
Someone, female or male, who has a depression (aka sag) in either their breasts or hindquarters. It is usually used towards younger people, as old people tend to sag most of the time, and it is generally assumed that they sag anyway.

One who sags.
See that sagster over there? Man, her bra's really holding her up, if that wasn't there, she'd be dragging along the floor.
by Alfons Richter April 01, 2008