-ness suffix etymology

Solution 1:

This suffix is widespread among the Germanic languages, though it is no longer productive in all:

• English -ness (Old English -nes(s), -nis(s), -nys(s))
• Dutch -nis (Old Dutch -nussi, -nisse, -nesse)
• German -nis (Old High German -nissi, -nessi, -nassi, -nussi)
• Gothic -nassus

There is no consensus yet about why the first vowel varies so widely among the Germanic languages.

The -n- was originally not part of the suffix, but of the stem of many root words that *-assus was often attached to; it was later stolen by the suffix and became part of it, which already happened in Proto-Germanic.

The -s is probably the nominative ending (my own, reasonable interpretation of the Woordenboek der Nederlandsche Taal).

That leaves *-assu- as the basic suffix (the WNT confirms this). It in turn came from *-ad-tu-: -ad- is a Germanix suffix forming verbs out of adjectives (Gothic -at(j?)-), and -tu- is a suffix forming abstract nominal stems from verbal stems.

The suffix -esse in French (fin-esse etc.) is not directly related: it comes from Latin -itia (fin-itia). The -ia part is probably the common Proto-Indo-European suffix *-ia/ya indicating mass nouns; it is possible that the -(i)t- part is related to Germanic -tu- in *-ad-tu-, but that seems doubtful.

Solution 2:

Oxford Dictionary Online states that it is of Germanic origin:

http://oxforddictionaries.com/definition/-ness?q=-ness