I don't understand Cockney rhyming slang. Can an individual make it up himself on the spot or does it have to be agreed upon beforehand by a number of people?
Each one must have started life as the invention of an individual but, if sufficiently memorable, it would catch on and eventually become part of the canon of slang expressions. Although some Cockney rhyming slang terms go way back to the early Victorian era, others are being added to the lexicon all the time. Recent examples might be "Ruby Murray = curry", "Vera Lynn = gin", "Barney Rubble = trouble", and "Pete Tong = wrong" (e.g. "it's all gone a bit Pete Tong").
I was told once that TWIT was an acronym for That's What I Thought
I just checked in the full Oxford English Dictionary, and it doesn't offer any origin for the word at all.
While I was there, I looked up "twat" as well. It, too, is of unknown or uncertain etymology, but I had heard at school that "twat" was another word for a nun's head-gear. This is actually mentioned in the OED, but only to say that it's incorrect; the entry in the OED shows that the original quote (from 1660) was clearly using the "rude" definition, but Browning (whoever he was) evidently took it to mean "nun's head-gear" due to the mention, in the same verse, of a cardinal's hat:
Erroneously used by Browning in 1727... under the impression that it denoted some part of a nun's attire. Vanity of Vanities, 1660: "They talk't of his having a Cardinall's Hat / But they'd send him as soon an Old Nun's Twat"
I'm so sure about "twat" meaning a nun's headgear (also called a wimple), because "an old nun's twat" fits well with nun meaning prostitute.
See also Shakespeare's "get thee to a nunnery" in which he may be referring to a brothel, not a convent.
--------------- Von Konigswald: Jack the Ripper plays shuffleboard. -- Happy Birthday, Wanda June by Kurt Vonnegut, c.1970.