From Proto-Finnic *-uc. The declension has been massively reworked by analogy:
- The oblique plural forms originally declined like those of the käsi type, but were replaced by plurals in -ks- from the vastaus-type declension, formed on the analogy of the suffix -s.
- Words that had acquired a nominal sense early on have crossed over entirely to the vastaus declension: e.g. sisarus (“sibling”), originally a semantic extension from *sisarus > sisaruus (“siblinghood”); vanhus (“old person”), originally a semantic extension from *vanhus > vanhuus (“oldness”).
- The long vowel appearing after consonants originates from a contraction *-ude- > *-ue- > -uu- in the genitive singular and nominative plural of bisyllabic stems, in which the third-syllable unstressed e was assimilated. This was generalized to other case forms of such words as well.
- In trisyllabic stems such as korkea → korkeus, rakas → rakkaus, the suffix would in all inflected forms gain secondary stress, preventing the loss of *-d-. From here *-d- was then reintroduced in the gen. sg. and nom. pl. of bisyllabic stems.
- Root-final consonants in trisyllabic stems were by contrast often lost (e.g. *korkeduden > korkeuden, *rakkahuden > rakkauden), leading to the regular retention of short -u- especially after vowels.
- The rare exceptions to this such as sisar → sisaruus have however also acquired the long-vowel allomorph.
From the Proto-Indo-European perfect active participle *-wós.