The presence of *-u- in the second syllable is aberrant, as it was not present in the PIE root or even in the related Germanic verb *melkaną. However, the evidence of all descendants speaks unambiguously in favour of its presence. Aside from being directly attested in most of them, it is also implied by Old Norse u-mutation in case forms where it would not be expected, and in the raising of e to i before u in the non-Ingvaeonic West Germanic languages.
Possible theories of origin of the disputed *-u-:
- According to Kümmel, the vowel *u is an anaptyctic vowel, inserted after the resonant in to ease the complicated cluster of three consonants *VRCs#.
- According to Szemerenyi (1992: 1125) it is the result of contamination of the full- and zero-grade in an ablauting paradigm */melk-z/, genitive */mulk-iz/, but this leaves the Schwebeablaut unexplained.
- According to Bammesberger (1990: 196f) this */u/ represents a schwa that arose between the */l/ and the */k/ in the nominative */melk-z/. However, this solution fails to explain why the same did not happen in e.g *alhs (“temple”).
- According to Kroonen, *-u- originates from the strong verb *mel(u)kaną (“to milk”), whence this archaic root noun likely derives. This does not answer the ultimate origin of the u however.
- Old English: meoluc, meolc, milc
- Old Frisian: melok
- Old Saxon: miluk
- Old Dutch: miluk, miloch
- Old High German: miluh
- Old Norse mjǫlk
- Gothic: 𐌼𐌹𐌻𐌿𐌺𐍃 (miluks)
- → Proto-Samic: *mēlkkē (borrowed)
- →? Proto-Slavic: *melko (possibly, borrowed)
- Kümmel, M.J. (2004), Ungeklärtes *u neben Liquida in germanischen Nomina, in: A. Hyllested, A. R. Jørgensen (et al. eds.), Per Aspera ad Asteriscos. Studia Indogermanica in honorem Jens Elmegård Rasmussen sexagenarii Idibus Martiis anno MMMIV (Innsbrucker Beiträge zur Sprachwissenschaft 112), Innsbruck: Institut für Sprachen und Literaturen der Universität Innsbruck, 291-303. p. 298
- Kroonen, Guus (2013) Etymological Dictionary of Proto-Germanic (Leiden Indo-European Etymological Dictionary Series; 11), Leiden, Boston: Brill, page 364
- Szemerenyi, O. (1987-1992), Scripta minora, selected essays in Inda-European, Greek, and Latin. Innsbruck
- Bammesberger, A. (1990), Die Morphologie des urgermanischen Nomens. Heidelberg