From Proto-Indo-European *péruti.
The form and history of this term is subject to controversy due to providing major difficulties in the traditional theory of the rise of the absolute and conjunct inflection in Old Irish. The traditional theory requires an early apocope of the final *-i that appeared in multiple Celtic personal endings, especially the third-person endings *-ti and *-nti that was blocked by a supposed transphonologized enclitic particle. The apocopated verbal forms are assumed by the theory to become Old Irish conjunct forms, while the encliticized forms became absolute forms. *ɸeruti is a major counterexample to the apocope rule the theory relies on, since the palatalization of the Old Irish descendant uraid indicates a word-final *-i remaining in an identical position to the verb-final *-i whose apocope is sought.
Schrijver and McCone, supporters of the traditional theory, attempt to eliminate this counterexample by deriving this adverb from an accusative *ɸerutam, creating an accusative with no fellow Indo-European parallel, citing what appears to be the accusative definite article being always being prefixed before uraid in Old Irish.[1] Kortlandt rejects extrapolating the accusative beyond Old Irish due to other Indo-European descendants of *peruti having long been fossilized, viewing such a defossilization of a clearly fossilized adverb as an unlikely development.[2] Later on, Matasović joined Kortlandt in rejecting it for semantic reasons, believing that a locative with *-i was more semantically appropriate for an expression of time.[3]
*ɸeruti[3]