The opening conversation in The Hobbit between Bilbo Baggins and Gandalf the wizard includes the following exchanges:
“Good morning!” said Bilbo, and he meant it.
But Gandalf looked at him from under long bushy eyebrows that stuck out further than the brim of his shady hat. “What do you mean?” be said. “Do you wish me a good morning, or mean that it is a good morning whether I want it or not; or that you feel good this morning; or that it is morning to be good on?”
“I beg your pardon, I haven’t asked for anything!”
“Yes, you have! Twice now. My pardon. I give it you.
The first chapter of Jespersen’s Philosophy of Grammar makes a distinction between formulas and free expressions that illuminates these exchanges.
Free expressions “have to be created in each case anew by the speaker, who inserts the words that fit the situation,” building up an expression by following a grammatical pattern or type (19).
Formulas cannot be changed. Jespersen gives as examples: ‘Good morning!’, ‘Thank you,’ and ‘Beg your pardon’ (18). Of such formulaic expressions he says:
One may indeed analyze such a formula and show that it consists of several words, but it is felt and handled as a unit, which may often mean something quite different from the meaning of the component words taken separately… (18-19)
So, from Jespersen’s perspective, the hobbit uses two formulaic expressions that Gandalf insists on hearing as free expressions, subverting Bilbo’s meaning by analysing the unanalysable.
Jespersen explains that free expressions follow regular grammatical patterns while irregular forms are always formulas (21). Once the formula coalesces, however, it may be assimilated into regular expression, e.g. by back-formation. To give one of Jespersen’s examples:
he breakfasts, breakfasted (formerly breaks fast, broke fast). (24)
Given that Gandalf proceeds to scratch a “queer sign” on Bilbo’s newly painted round front door that means ‘burglar,’ it is worth noting that the OED gives their first use of back-formation as the definition of to burgle (1889): “Late 19th century: originally a humorous and colloquial back-formation from burglar.” But while Bilbo is to become the burglar who will ultimately burgle Smaug, the narrator resists any back-formation with breakfast:
He had only just had breakfast
Gandalf, however, is more than happy to engage in back-formation, thereby demonstrating his understanding of the formulaic ‘Good morning!’:
To think that I should have lived to be good-morninged by Belladonna Took’s son, as if I was selling buttons at the door!