Old English hat "hot, flaming, opposite of cold," used of the sun or air, of fire, of objects made hot; also “fervent, fierce, intense, excited,” from Proto-Germanic *haita- (source also of Old Saxon and Old Frisian het, Old Norse heitr, Middle Dutch and Dutch heet, German heiß "hot," Gothic heito "heat of a fever"), of uncertain origin, perhaps related to Lithuanian kaisti "to grow hot;" both could be from a substratum word.