-nus

Etymology 1
From, from.

Usage notes
The suffix -nus appears in various derived adjectives, including relational adjectives to nouns such as, (from , ) or ,  (from , ).

In terms of etymology, it also occurs in various inherited adjectives or nouns (some derived from Proto-Indo-European verbal bases), such as the following:

Through rebracketing (e.g. reanalysis of tribū-nus, from the u-stem noun tribus, as trib-ūnus), it gave rise to a number of variant suffixes, such as  or, some of which show greater productivity in Latin than bare.

Etymology 2
Ultimately the same as the above, but apparently rebracketed at some point as *-sno- (extracted from words like *tris-no-, the original form of ).

Usage notes
In pre-Latin, the suffix (Classical Latin -nus) was attached to numeral adverbs, cardinal numerals, or numeral stems to form distributive numerals, such as
 * → (equivalent to ).
 * → →  →  (equivalent to ).
 * → →  →  →  (equivalent to ).
 * → (equivalent to ).

At some point in pre-Latin, the suffix seems to have become extended to *-sno-, presumably as the result of rebracketing of forms like *tris-no- as *tri-sno-, *dwis-no- as *dwi-sno- or *seks-no- as *sek(s)-sno-. By the time of Classical Latin, *-s- before *-n- had been lost by regular sound change, but it caused any preceding nasal or plosive consonants to be deleted and the preceding vowel to be lengthened. Thus, it can be inferred that the *-sno- variant of the suffix was used to form words like the following:

The -ēno- found in decades such as, probably developed from -ent-sno-.

Then the ending seems to have been extended from some of the above forms and used as a third allomorph of this suffix to form some of the other distributive numerals:
 * (alternative form of )
 * (alternative form of )

Descendants
Via :