Secondat: thing is

It's a term of law and governance from the Nordic, Germanic races. The Early English knew it as a council of elders, a court with its judges, a meeting of lord and thanes, and, from that meaning, the matter to be decided by the assembled men (always men of course) who did the deciding.

It's grown since then.

One source tells

The thing "what's stylish or fashionable" is recorded from 1762. Phrase do your thing "follow your particular predilection," though associated with hippie-speak of 1960s is attested from 1841. Used colloquially since 1602 to indicate things the speaker can't name at the moment, often with various meaningless suffixes, e.g. thingumbob (1751), thingamajig (1824).

And from OED:

'to see his things'
1550 in Acts Privy Council — The Lord Admirall desired licence to go into Lincolnshire for a moneth to see his thinges that he had not seen of a long tyme.

'go you thing'
1598 SHAKESPEARE Henry IV, Pt. 1 — Go you thing, go.

'my thing is'
1652 J. FLETCHER Wild-goose Chase — Well, there is something, Sister. Or. If there be, Brother, 'Tis none of their things, 'tis not yet so monstrous; My thing is Mariage.

'quite the thing'
1775 F. BURNEY Diary — Mr. Bruce was quite the thing; he addressed himself with great gallantry to us all alternately.

'things are in a good way'
1776 S. FOOTE Bankrupt — Never fear, things are in a very good way.

'such a thing'
1813 J. AUSTEN Pride & Prejudice — It would have been such a thing for me! The quiet, the retirement of such a life, would have answered all my ideas of happiness!

'I'll tell you...'
1831 T. C. GRATTAN Jacqueline of Holland — I'll tell you a thing, Bishop Zweder; you know as little of the bold candour of chivalry as this English earl does of the guile of priestcraft.

'know (or not know) a thing'
1844 R. W. EMERSON New Eng. Reformers in Ess. 2nd Ser. — We are shut up in schools, and colleges, and recitation-rooms, for ten or fifteen years, and come out at last with a bag of wind, a memory of words, and do not know a thing.

'the thing had clean disappeared'
1893 R. L. STEVENSON Catriona — Wi' the bang and the skirl the thing had clean disappeared.

'old man, the thing is'
1915 J. GALSWORTHY Freelands — Look here, old man, the thing is, of course, to see it in proportion.

'done a thing (or not)'
1956 M. DICKENS Angel in Corner — I haven't done a thing all day, and I'm as tired as a dog.

'that poetry thing'
1906 ‘H. MCHUGH’ Skiddoo! — When it comes to that poetry thing he thinks he can make Hank Longfellow beat it up a tree.

'not my thing'
1936 G. B. STERN Monogram — If pottery's your thing. Mountains are not my thing. The sea is my thing.

'make a thing'
1952 E. GRIERSON Reputation for Song — Steady on, Laura... Don't let's make a thing of it.

'having a thing with'
1959 P. D. CUMMINS tr. D. Dolci Rep. from Palermo (U.S. ed.) — One of my pals..found out he was having a thing with a gorgeous blonde.

'a black thing'
1967 N.Y. Times — Few whites are journeying to Harlem for entertainment. ‘It's a black thing now... It's by blacks and for blacks and you don't see many whites up here.’

'boy things'
1983 M. MACKIE Exploring Gender Relations — Similarity provides a basis for shared activities, in this case, doing ‘boy things’ or ‘girl things’.

'a guy thing'
1991 J. PHILLIPS You'll never eat Lunch in this Town Again — I entertain us both with a brief negotiation, not something I care to do, but I know if I don't he'll think I'm a wuss and feel compelled to rip me off. Not his fault. It's a guy thing.

'a thing for books'
1994 N.Y. Times Bk. Rev. — Sally Beaumann clearly has a thing for fat books.

'a thing for my friend'
2006 Cosmopolitan — I'd had a thing for my friend Jon since I'd met him two years earlier, but because we were part of a group of friends, we'd done nothing more than flirt.

Then there are

'wild things'
'a queer thing'
'a funny thing'
'the least thing'
'thinking things over'
'get things done'
'take things as they come'
'from the beginning of things'

From my current read:

'praying things to come out'
The spontaneity and savage freedom of the plant life in this land of alternate hot sun and warm showers at last blurred and made insignificant to me the men who braved it in silk hats and broadcloth there, and the trains, and the jewellers, shops, for my experience of vegetation was got on my knees in a London suburb, praying things to come out of the cold mud. Here, I began to suspect, they besieged us, quick and turbulent, an exhaustible army, ready to reconquer the foothold man had hardly won, and to obliterate his works.

'beat our poor weak bodies against an adamant thing'
Our captain is a very excellent master mariner, but occasionally he likes to test the security of his absolute autocracy, to see if it is still sound. I never knew it when it was not; but yet he must, to assure himself of a certainty, or to exercise some devilish choler in his nature, sometimes beat our poor weak bodies against the adamant thing, to see which first will break. I will say for him that he is always polite when handing back to us our bruised fragments. Here he was giving us a day's freedom, and one's first city of the tropics in which to spend it; and we agreed with him that such a waste of time was almost unbearable, and left hurriedly.

There's a verb form, as in De-Thinging Thursday, or this mystery:

The Taoists constantly said, "To thing things, but not to be thinged by things." The things which assist the transforming and nourishing processes, these are not merely in the category of things (i.e., not to be thinged by things). The distinction between those two classes consists in whether they have enlightenment or not.
-- The Spirit of Chinese Philosophy By Fung Yu-LAN

And I've already quoted the Cooper Square duo:

Girl: Aaron! Hi!
Guy: Hey! Are you going to the thing?
Girl: Yeah, the thing!
(guy starts walking away)
Girl: Wait, Aaron! Hold up!
Guy: What?
Girl: Did you see that puppy?
Guy: Yeah, I pointed at it and laughed at it a few times.

And from the same source: