(automotive) The horizontal line just below the windows of a car or other vehicle.
1979 August, Graham Burtenshaw, Michael S. Welch, “O.V.S. Bulleid's SR loco-hauled coaches - 1”, inRailway World, page396:
[...] with the adoption of a curved bodyside profile Bulleid considerably raised the height of thewaistline and at the shoulder height of seated passengers, just where the extra space was most needed.