The phrase "belt line" refers to an imaginary line that runs horizontally around your waist or midsection. It is commonly used to describe the area where you typically wear your belt or where the waist of your pants or skirt sits. Full definition
It has a high belt line with chunky fenders. (autoline.tv)
The rear flows as nicely as the cars lines, with the trunk lid and bumpers connecting to the flowing rear glass and rising lower belt line. (thesupercars.org)
I think it takes away from the shoulder and belt line of the vehicle. (autoline.tv)