-in 1
a suffix, occurring in adjectives of Greek and Latin origin, meaning "pertaining to," and (in nouns thence derived) also imitated in English ( coffin; cousin, etc.).
[ ME -in, -ine -inus, -ina, -inum -inos, -ine, -inon ]
-in 2
a noun suffix used in a special manner in chemical and mineralogical nomenclature ( glycerin; acetin, etc.). In spelling, usage wavers between -in and -ine. In chemistry a certain distinction of use is attempted, basic substances having the termination -ine rather than -in ( aconitine; aniline, etc.), and -in being restricted to certain neutral compounds, glycerides, glucosides, and proteids ( albumin; palmitin, etc.), but this distinction is not always observed.
[ -ina. See -INE 2 ]
-in 3
a suffixal use of the adverb in , extracted from sit-in , forming compound nouns, usually from verbs, referring to organized protests through or in support of the named activity ( kneel-in; chain-in; be-in ) or, more generally, to any organized social or cultural activity ( cook-in; sing-in ).