Entries linking to setness
"Action of hardening," 1837; also "manner or position in which something is set" (1530s), hence "general movement, direction, tendency" (1560s); "build, form" (1610s), hence "bearing, carriage" (1855); "action of fixing the hair in a particular style" (1933).
"Something that has been set" (1510s), hence the use in tennis (1570s) and the theatrical meaning "scenery for an individual scene in a play, etc.," recorded from 1859. Other meanings OED groups under "miscellaneous technical senses" include "piece of electrical apparatus" (1891, first in telegraphy); "burrow of a badger" (1898). Old English had set "seat," in plural "camp; stable," but OED finds it "doubtful whether this survived beyond OE." Compare set (n.1).
Set (n.1) and set (n.2) are not always distinguished in dictionaries; OED has them as two entries, Century Dictionary as one. The difference of opinion seems to be whether the set meaning "group, grouping" (here (n.2)) is a borrowing of the unrelated French word that sounds like the native English one, or a borrowing of the sense only, which was absorbed into the English word.
word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).