A semmit (or simmit as we pronounced it in Paisley) is merely the Scotch word for an undershirt or vest. I don't ever remember a string vest being referred to as a semmit, mind you we didn't have string vests when I was a boy ... brown paper between body and shirt kept out the cold! The word itself, semat, applied to a Roman tunic. There is also a connection with samite which was a silk fabric. Very interesting.