This refers to pants drafted or sewn with insufficient ease in the posterior, causing the fabric to stretch tightly across the buttocks. It often results from a crotch curve that is too shallow or a back rise that is too short for the wearer. While sometimes an intentional style choice, in tailoring it usually indicates a fit defect requiring a pattern adjustment.
Heard on the job
These sample jeans are way too tight-assed, we need to scoop out the back crotch seam by at least half an inch.