To the very hot weather that comes so often at the end of July and the beginning of August, the traditional name of "dog-days" has been given, almost from time immemorial. In recent years the name has given rise, in the popular mind at least, to the idea that dogs are much more liable to go mad during these months than at any other season of the year. The term dog-days, however, is only our English equivalent for the Latin phrase, the days of the dog star, when Sirius, called by the Romans also Canis, the dog, is in the ascendant—the heliacal rising of this star occurring between July 20 and August 15.
Words are usually supposed merely to convey ideas; as a matter of fact they sometimes dictate a new sense that is totally unjustified by their etymology, but that often usurps the place of the original signification intended.