From Pre-Proto-Indo-European *méh₂ters.
In older literature usually reconstructed de-laryngealized as *mātēr and explained as a combination of a nursery word *ma (“mother”) and the agentive suffix *-tēr. However, Balto-Slavic forms have acute accent and require a root laryngeal and a short vowel, reflecting *méh₂- (as opposed to *mḗh₂- which would've yielded a circumflexed vowel). Also, no trace of a lengthened grade in the root can be found: in no language did Eichner's law operate (which predicts non-coloration of the *ē in *mḗh₂-tr).
There is evidence for both suffixal and radical accent:
- Greek μήτηρ (mḗtēr) shows accentuation on the root
- Germanic *mōdḗr (Verner's law) and Sanskrit माता (mātā́) show accentuation on the suffix
- Balto-Slavic is inconclusive (the original accent could have been retracted to the root syllable by Hirt's law).
Only one of these can be the original accentuation, which means that the other must have undergone an analogical accent shift. PIE kinship terms that are semantically closest to "mother" are *ph₂tḗr (“father”) and *dʰugh₂tḗr (“daughter”) and they show in their nominative singular forms accentuation of the suffix, which indicates that the word for "mother" was likely originally accented on the root, shifting its accent to the suffix in some languages under the influence of words for "daughter" and "father".
The reconstruction of paradigm is similarly problematic:
- accusative singular forms are: Sanskrit मातरम् (mātáram), Ancient Greek μητέρα (mētéra), i.e. accented on the suffix only (Lithuanian móterį has radical stress due to Hirt's law)
- genitive singular forms: Sanskrit मातुः (mātúḥ), Ancient Greek μητρός (mētrós), i.e. accented only on the desinence only (Lithuanian móteres has radical stress due to Hirt's law)
Strictly methodologically this would point to a hysterokinetic inflection, with the reconstructed accusative singular *meh₂térm and the genitive singular *meh₂trés. However, since the words for "father" and "daughter" influenced the nominative singular form in some languages, it is possible that the same analogical process occurred in the accusative and genitive singular forms as well. The conclusive evidence for the original acrostatic inflection is the Sanskrit ending for the genitive singular in the word मातुः (mātúḥ), which is -उः (-uḥ), the same ending used for other kinship terms: पितुः (pitúḥ, “father (gen. sg.)”), दुहितुः (duhitúḥ, “daughter (gen. sg.)”), भ्रातुः (bhrā́tuḥ, “brother (gen. sg.)”) etc. This ending can only reflect *-C-r-s, with zero-grade of the suffix as well as of the ending, which is only possible in an acrostatic paradigm. A similar ending is found in Old Icelandic, where all kinship terms show the ending -or < PIE *-C-r-s, such as fǫðor (“father (gen. sg.)”), móðor (“mother (gen. sg.)”), bróðor (“brother (gen. sg.)”), dóttor (“daughter (gen. sg.)”). However, both in Sanskrit and in Old Icelandic this ending is found in all kinship terms, even in the ones that we know for sure not to have been acrostatically inflected like *ph₂tḗr (“father”) (genitive *ph₂tr̥és >> Sanskrit पितुः (pitúḥ), Old Icelandic fǫðor) and *dʰugh₂tḗr (“daughter”) (genitive *dʰugtr̥és >> Sanskrit दुहितुः (duhitúḥ), Old Icelandic dóttor). It follows that the ending must have spread from some other kinship term. Since PIE *bʰréh₂tēr (“brother”) must have been acrostatically inflected, the endings -उः (-uḥ) and -or must have been original in this word (*bʰréh₂tr̥s > Sanskrit भ्रातुः (bhrā́tuḥ), Old Icelandic bróðor). Given that it is unlikely that on the basis of the word for "brother" alone the ending *-r-s would in these languages have spread so vastly, even to the word for "father", we must assume that the ending was also present in another kinship term, namely the word for "mother", which was (as explained above) in its nominative singular form radically stressed, which would fit the acrostatic accentuation pattern, being the source of source of this genitive singular ending *-r-s.
*méh₂tēr f (oblique stem *méh₂tr-)
- Albanian: motër "sister" (< dial. motrë < *mātrā < *méh₂treh₂ "mother's sister")
- Balto-Slavic: *māˀtē (see there for further descendants)
- Slavic: *mati (see there for further descendants)
- Germanic: *mōdēr (see there for further descendants)
- Hellenic: *mā́tēr (see there for further descendants)
- Indo-Iranian: *mā́tar- < *maHtār- (see there for further descendants)
- Italic: *mātēr (see there for further descendants)
- Latin: māter (see there for further descendants)
- Phrygian: ματαρ (matar)
- Tocharian: *mācer
- Andrew L. Sihler (1995) New Comparative Grammar of Greek and Latin, New York, Oxford, Oxford University Press