the adverb in attached to a verb as a word-forming element, by 1960, abstracted from sit-in, which is attested from 1941 in reference to protests and 1937 in reference to labor union actions (which probably was influenced by sit-down strike) but was popularized in reference to civil disobedience protests aimed at segregated lunch counters.
As a word-forming element it was extended first of other types of protests, then by 1965 to any sort of communal gathering (such as love-in, attested by 1967). In labor actions it was perhaps less useful: "a mass of workers calling in sick to absent themselves in protest" was called both a sick-out (1970) and a sick-in (1974).
word-forming element making nouns of quality, state, or condition, a confusion in English of three similar suffixes from Latin:
1. in primacy, etc., from Old French -acie and directly from Medieval Latin -acia, Late Latin -atia, making nouns of quality, state, or condition from nouns in -as.
2. in advocacy, etc., from Late Latin -atia, forming nouns of state from nouns in -atus.
3. in fallacy, etc., from Latin -acia, forming nouns of quality from adjectives in -ax (genitive -acis). Also forming part of -cracy. It has been extended in English to nouns not found in Latin (accuracy) and to non-Latin words (piracy).
word-forming element, from Latin -farius, -fariam "in (so many) parts," as in bifariam "in two parts or places, in two ways;" multifariam "in many places," an element of disputed origin. Watkins suggests it is from PIE *dwi-dhe- "making two," from roots *dwi- "two" + *dhe- "to put, set." It also has been derived from Latin fari "to say" (as in nefarious), but de Vaan writes that "the alleged semantic development to 'in n ways' is obscure," and he points to the suggestion of a PIE *-dho-, with cognates in Sanskrit dvidha (adv.) "twofold;" tridha "threefold."
In paraphernalia, Mammalia, regalia, etc. it represents Latin or Greek -a (see -a (2)), plural suffix of nouns in -ium (Latin) or -ion (Greek), with formative or euphonic -i-.
Middle English -ik, -ick, word-forming element making adjectives, "having to do with, having the nature of, being, made of, caused by, similar to," from French -ique and directly from Latin -icus or from cognate Greek -ikos "in the manner of; pertaining to." From PIE adjective suffix *-(i)ko, which also yielded Slavic -isku, adjectival suffix indicating origin, the source of the -sky (Russian -skii) in many surnames. In chemistry, indicating a higher valence than names in -ous (first in benzoic, 1791).
In Middle English and after often spelled -ick, -ike, -ique. Variant forms in -ick (critick, ethick) were common in early Modern English and survived in English dictionaries into early 19c. This spelling was supported by Johnson but opposed by Webster, who prevailed.
as in beatnik, etc., suffix used in word formation from c. 1945, from Yiddish -nik (as in nudnik "a bore"), from Russian -nik, common personal suffix meaning "person or thing associated with or involved in" (compare nudnik; kolkhoznik "member of a kolkhoz"). Rocketed to popularity with sputnik (q.v.), hence its brief vogue in English word-formation, as in robotnik "person behaving with mindless obedience" (1960).