Unknown. Several possibilities have been proposed, although none are convincing:
- Relation to Proto-Indo-European *wódr̥ (“water”), whence also Proto-Germanic *watōr.
- Relation to Proto-Celtic *windos (“white”).
Most descendants (in particular Gothic) show a u-stem paradigm:
However, Old Norse shows a consonant stem paradigm:
If that is indeed the original paradigm, it may have arisen as follows: The nominative and vocative singular would have come from older *wintr̥-s and *wintr̥, respectively, into which an epenthetic u had been inserted before a syllabic sonorant, as is usual in Germanic. This formed *winturz and *wintur. The -ur- was then later metathesized into a form resembling a u-stem nominative (likely based on the accusative forms).
- Old English: winter
- Old Frisian: winter
- Old Saxon: wintar
- Old Dutch: *wintar
- Old High German: wintar
- Old Norse: vetr, vintr
- Gothic: 𐍅𐌹𐌽𐍄𐍂𐌿𐍃 (wintrus)