One of those names so often met with among the Latins, which have had their origin in some personal peculiarity.  It means soft-haired or downy-cheeked, from iulus, derived from the Greek ioulos, signifying the downy or soft hair of early youth.  The name is said to have been assumed by Ascanius of Troy when he, while in early manhood, came off the victor in a combat.
     Some derive the name Iulus from Ilium, the citadel of Troy, and quote in support of their theory the line Julius a magno demisum nomen Julo [Julius, a name descended from the great Julius], Julius or Ascanius having been the son of Aeneas and Lavinia.
     Certain it is, that the Julian gens was older than Rome itself, and one of its members bore the cognomen of Julus, namely, Aeneas' son.
     The family was not originally a Roman one, and it did not migrate to Rome until the time of Tullus Hostilius, the third and somewhat mythical King of Rome, the successor of Numa, and, though the Julian family was a noble one, it was undistinguished until it burst into glory in Caius Julius Caesar.
     After the time of Julius Caesar every family that had a representative who ascended the Imperial throne was supposed to be adopted into the Julian gens or clan, and thenceforward Julius, Julian, and Julia became common names wherever the wide-embracing arms of the Roman Empire reached. 
     England at once had her full quota of Julius's, and in Wales, especially in the form of Jolo, it lingered on.  In England it never became obsolete, and during the eighteenth century, when nothing was thought beautiful that was not classical, it enjoyed a vigorous revival in both its masculine and its feminine forms. (Girls' Christian Names, Swan, 1905)

"At puer Ascanius, cui nunc cognomen Iulo,
Additur Ilus erat dum res stetit Ilia reguo."

"The boy Ascanius, now Iulus named—
Ilus he was while Ilium's realm still stood,"

quoth Jupiter, in the first book of the Æneid, whence Virgil's commentators aver that Ascanius was at first called after Ilus, the river that gave Troy the additional title of Ilium; but that during the conquest of Italy he was termed Iulus, from ιουλος (the first down on the chin), because he was still beardless when he killed Mezentius.   The father of gods and men continues:

"Nascetur pulchrâ Trojanus origine Cæsar,
(Imperium Oceano, famam qui terminet astris,)
Julius, a magno nomen Iulo."

"A Trojan, by high lineage shall arise—
Cæsar (whose conquering fame the sea and stars shall bound),
Called Julius, from Iulus, mighty name."

The Julian gens certainly exceeded Rome in antiquity, and one of their distinguished families bore the cognomen of Iulus; but in spite of Jupiter and Virgil, Livy makes Iulus, or Ascanius, not the Trojan son of Æneas and the deserted Creusa, but the Latin son of Æneas and Lavinia, and modern etymologists hazard the conjecture that Julus may be only a diminutive of dius (divine), since the derivation of Jupiter from Deus pater (father of gods) proves that such is the tendency of the language.
     The family resided at Alba Longa till the destruction of the city by Tullus Hostilius, and then came to Rome, where, though of very high rank, they did not become distinguished till, once for all, their star culminated in the great Caius Julius Cæsar, after whom the Julii were only adoptive, though Julia was the favourite name of the emperors daughters, and their freedmen and newly-made citizens multiplied Julius and Julianus throughout the empire. (History of Christian Names, Yonge, 1884)