Final answer:
The phrase 'blue-jeaned sitter' refers to the thief's backside covered in blue jeans, and a synonym would be 'backside' or 'butt'.
Step-by-step explanation:
A synonym for "blue-jeaned sitter" is backside or butt. The phrase 'blue-jeaned sitter' itself is a colorful description merging clothing with a body part and it implies the backside covered by blue jeans. In the context given, it is clear that the large woman retaliated by kicking the would-be-thief in the rear-end adorned in blue jeans.