From Latin labii, from the first word of the sixth line of Ut queant laxis, the medieval hymn on which solfège was based because its lines started on each note of the scale successively.
Sound used to form meaningless song refrains. Of imitative origin. Compare Old English lā, Ancient Greek λαλαγε ("babble"), German lallen ("to babble").
From Middle English la, from Old English lā. More at lo.