-in¹

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.).

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.

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).