Shortening of 'seamed stockings', vintage or reproduction nylon or sometimes silk hosiery with a seam going up the back of the leg from under the toes to the stocking top, or welt. The stockings should be shaped to the leg (fully-fashioned) and are sewn up the back, hence the seam. Modern seams are often purely aesthetic and are not neccessary to the construction of the stocking.
Phwoar! Look at that wiggle! She looks good in that tight skirt with seams 'n' heels.