Middle English thing, from Old English þing, þingc "meeting, assembly, council, discussion," also "action, deed to be done." In late Old English, "concrete inanimate object; that which exists by itself; entity, being, creature;" also "event."
The sense evolution probably is from the notion of the "matter" or subject of deliberation in an assembly. Compare French chose, Spanish cosa "thing," from Latin causa "judicial process, lawsuit, case" (see cause (n.)); Latin res "affair, thing," also "case at law, cause."
It is reconstructed to be from Proto-Germanic *thinga- "assembly" (source also of Old Frisian thing "assembly, council, suit, matter, thing," Middle Dutch dinc "court-day, suit, plea, concern, affair, thing," Dutch ding "thing," Old High German ding "public assembly for judgment and business, lawsuit," German Ding "affair, matter, thing," Old Norse þing "public assembly").
The Germanic word is perhaps (Watkins, Boutkan) literally "appointed time," from a PIE *tenk- (1), from root *ten- "stretch," perhaps on notion of "stretch of time for a meeting or assembly."
The sense of "meeting, assembly" disappeared by early Middle English but is preserved in second element of hustings and in Icelandic, as in Althing, the nation's general assembly.
In reference to a living creature or person by early 12c., often affectionately or pityingly (young thing is from c. 1200). Thing has been used colloquially since c. 1600 to indicate what inanimate object the speaker can't name at the moment, often with meaningless elaborating suffixes (see thingamajig).
Related: Things (c. 1300 as "personal possessions"). Adjective thingal (1857) is rarely used. The thing "what's stylish or fashionable" is recorded from 1762. The phrase do your thing "follow your particular predilection," though associated since 1960s with hippie-speak, is attested from 1841 (Emerson).
