suffix in pet proper names (such as Johnny, Kitty), first recorded in Scottish c. 1400; according to OED it became frequent in English 15c.-16c. Extension to surnames seems to date from c. 1940. Use with common nouns seems to have begun in Scottish with laddie (1546) and become popular in English due to Burns' poems, but the same formation appears to be represented much earlier in baby and puppy.
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). 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).