word-forming element meaning "a speaking, discourse, treatise, doctrine, theory, science," from Greek -logia (often via French -logie or Medieval Latin -logia), from -log-, combining form of legein "to speak, tell;" thus, "the character or deportment of one who speaks or treats of (a certain subject);" from PIE root *leg- (1) "to collect, gather," with derivatives meaning "to speak (to 'pick out words')." Often via Medieval Latin -logia, French -logie. In philology "love of learning; love of words or discourse," apology, doxology, analogy, trilogy, etc., Greek logos "word, speech, statement, discourse" is directly concerned.
before vowels phil-, word-forming element meaning "loving, fond of, tending to," from Greek philos (adj.) "dear, loved, beloved," as a noun, "friend," from philein "to love, regard with affection," a word of unknown origin. Productive of a great many compounds in ancient Greek (such as philokybos "a lover of dice-play"). Opposed to miso-. Compare -phile.
also -phil, word-forming element meaning "one that loves, likes, or is attracted to," via French -phile and Medieval Latin -philus in this sense, from Greek -philos, common suffix in personal names (such as Theophilos), from philos "loving, friendly, dear; related, own," related to philein "to love," which is of unknown origin. According to Beekes, the original meaning was "own, accompanying" rather than "beloved."
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; slim-in, for dieters, 1973). 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).