Grannus (also Granus, Mogounus, and Amarcolitanus) was a Celtic deity of classical antiquity. He was regularly identified with Apollo as Apollo Grannus and frequently worshipped in conjunction with Sirona, and sometimes with Mars and other deities. The theonym Grannus is a latinized form of Gaulish Grannos. The same stem appears in the personal names Grania, Grannia, Grannicus, and Grannica, as well as in the place names Grignols (from an earlier *Granno-ialon 'Grannus' clearing'), Aquae Granni (> Aachen), and Granéjouls. Its etymology remains debated. The name could be connected to Proto-Celtic *grand-/grend-, meaning 'beard' (cf. Middle Irish grend, Middle Welsh grann 'chin, beard, cheek', Middle Breton grann 'eyebrow'), although some scholars have pointed that the god is never actually portrayed with a beard. Old French grenon ('small beard'), Old Spanish greñon ('beard') and Occitan gren ('moustache') are derived from an earlier *grennos, that is supposed to be Gaulish, but the vocalism is difficult to reconcile with the other forms. An alternative etymology connects the name to a reconstructed form *gra-snó- (< *gwhr-snó-), which could be related to Proto-Celtic *gwrīns-/gwrens-, meaning 'heat' (cf. Middle Irish grīs 'heat, glow, embers', Middle Welsh gwres 'heat [of the sun, fire], passion, lust'). Scholar Jürgen Zeidler contends that this would be a "probable reference to the sun's heat and its healing properties". In early twentieth century scholarship, the theonym was often compared with the Old Irish grían ('sun'), which, according to linguist Ranko Matasović, should be derived from Proto-Celtic *gwrensā (> Primitive Irish *gwrēnā). At Monthelon, Grannus is called Deus Apollo Grannus Amarcolitanus ("The one with a piercing or far-reaching look"), and at Horbourg-Wihr Apollo Grannus Mogounus. In all of his centres of worship where he is assimilated to a Roman god, Grannus was identified with Apollo, presumably in Apollo's role as a healing or solar deity. In Trier, he is identified more specifically with Phoebus as Apollo Grannus Phoebus.