From hōc + diē (locative singular), literally “on this day”; a construction found in Old Latin, and also used in crāstinī diē (“tomorrow”, literally “on tomorrow's day”). Compare Welsh heddiw, Breton hiziv, German heute (“today”), Russian сего́дня (sevódnja, “today”), which are semantically the same construction, but with etymologically unrelated roots, and hence are not cognates.