In historical linguistics, Italo-Celtic is a grouping of the Italic and Celtic branches of the Indo-European language family on the basis of features shared by these two branches and no others. There is controversy about the causes of these similarities. They are usually considered to be innovations, likely to have developed after the breakup of the Proto-Indo-European language. It is also possible that some of these are not innovations, but shared conservative features, i.e. original Indo-European language features which have disappeared in all other language groups. What is commonly accepted is that the shared features may usefully be thought of as Italo-Celtic forms.
Italo-Celts (I-C) homeland north of the Black Sea, according to one interpretation of the theory
The traditional interpretation of the data is that these two subgroups of the Indo-European language family are generally more closely related to each other than to the other Indo-European languages. This could imply that they are descended from a common ancestor, a Proto-Italo-Celtic which can be partly reconstructed by the comparative method. Those scholars who believe Proto-Italo-Celtic was an identifiable historical language estimate that it was spoken in the third or second millennium BC somewhere in south-central Europe. This hypothesis fell out of favour after being reexamined by Calvert Watkins in 1966. Nevertheless, some scholars, such as Frederik Kortlandt, continued to be interested in the theory. In 2002 a paper by Ringe, Warnow, and Taylor, employing computational methods as a supplement to the traditional linguistic subgrouping methodology, argued in favour of an Italo-Celtic subgroup, and in 2007 Kortlandt attempted a reconstruction of a Proto-Italo-Celtic.
The most common alternative interpretation is that the close proximity of Proto-Celtic and Proto-Italic over a long period could have encouraged the parallel development of what were already quite separate languages; areal features within a Sprachbund. As Watkins (1966) puts it, "the community of -ī in Italic and Celtic is attributable to early contact, rather than to an original unity". The assumed period of language contact could then be later, perhaps continuing well into the first millennium BC.
However, if some of the forms are archaic elements of Proto-Indo-European that were lost in other branches, neither model of post-PIE relationship need be postulated. Italic and especially Celtic also share some distinctive features with the Hittite language (an Anatolian language) and the Tocharian languages, and these features are certainly archaisms.
The principal Italo-Celtic forms are:
the thematic genitive in ī (dominus, dominī). Both in Italic (Popliosio Valesiosio, Lapis Satricanus) and in Celtic (Lepontic-oiso, Celtiberian-o), traces of the -osyo genitive of Proto-Indo-European (PIE) have also been discovered, which might indicate that the spread of the ī genitive occurred in the two groups independently (or by areal diffusion). The ī genitive has been compared to the so-called Cvi formation in Sanskrit, but that too is probably a comparatively late development. The phenomenon is probably related to the feminine long ī stems and the Luwiani-mutation.
the formation of superlatives with reflexes of the PIE suffix *-ism̥mo (Latin fortis, fortissimus "strong, strongest", Old Irish sen, sinem "old, oldest", Oscan mais, maimas "more, most"), where branches outside Italic and Celtic derive superlatives with reflexes of PIE *-isto- instead (Sanskrit: urús, váriṣṭhas "broad, broadest", Ancient Greek: 'καλός, κάλλιστος "beautiful, fairest", Old Norserauðr, rauðastr "red, reddest", as well as, of course, English "-est").
the ā-subjunctive. Both Italic and Celtic have a subjunctive descended from an earlier optative in -ā-. Such an optative is not known from other languages, but the suffix occurs in Balto-Slavic and Tocharian past tense formations, and possibly in Hittite-ahh-.
the collapsing of the PIE aorist and perfect into a single past tense. In both groups, this is a relatively late development of the proto-languages, possibly dating to the time of Italo-Celtic language contact.
the assimilation of *p to a following *kʷ. This development obviously predates the Celtic loss of *p:
PIE *penkʷe 'five' → Latin quīnque; Old Irish cóic
PIE *perkʷu- 'oak' → Latin quercus; Goidelic ethnonym Querni, in northwest Hispania Querquerni.
PIE *pekʷ- 'cook' → Latincoquere; Welshpobi (Welsh p presupposes Proto-Celtic *kʷ)
A number of other similarities continue to be pointed out and debated.
The r-passive (mediopassive voice) was initially thought to be an innovation restricted to Italo-Celtic until it was found to be a retained archaism shared with Hittite, Tocharian, and possibly the Phrygian language.
Winfred P. Lehmann, "Frozen Residues and Relative Dating", in Varia on the Indo-European Past: Papers in Memory of Marija Gimbutas, eds. Miriam Robbins Dexter and Edgar C. Polomé. Washington D.C.: Institute for the Study of Man, 1997. pp. 223–46
Schrijver, Peter (2015). "Pruners and trainers of the Celtic family tree: The rise and development of Celtic in light of language contact". Proceedings of the XIV International Congress of Celtic Studies, Maynooth 2011. Dublin: Dublin Institute for Advanced Studies. pp. 191–219.