“Oengus” in Dictionary of the Irish Language, Royal Irish Academy, 1913–76.
Etymology
From Middle IrishAengus, Oengus, from Old IrishOíngus, from oín(“one”), from Proto-Celtic*oinos, from Proto-Indo-European*óynos(“single, one”). The etymology of the second element is disputed; most likely it is Old Irish gus(“strength, vigour”), from Proto-Celtic *gustu-, or it may be from Proto-Celtic *gus-(“choose”).