Boiled Dressing for Cold Slaw

Boil one-half cup vinegar with two teaspoons

From 1911:

Boil one-half cup vinegar with two teaspoons sugar, half a teaspoon each salt, mustard and a little pepper; rub one fourth cup of butter to a cream with one teaspoon flour, and pour the boiling vinegar on it.
Cook five minutes and pour over one well beaten egg.
Allow it to cool before using.