Presumably from ayyānu ‘where?’. If this derivation is correct, the Akkadian form cannot be an immediate cognate of PC *ˀayn)- (against most of the standard dictionaries and Faber 1991:414), but a result of a parallel semantic development. Indeed, the negative meaning of (ay)yānu does not seem to be attested before Middle Babylonian and, consequently, has no chances to be inherited from PS: a supposed archaism of this kind is unlikely to be so completely missing from the extant OB corpus (where this meaning is expressed analytically by ul ibašši)
(Kogan 2015: 281)