You ask : ‘Is this just a peculiar quirk of Mr. Jaggers' character, or does the gesture have some meaning?’
The answer I contend is it is a quirk of character, but in addition has specific significance and meaning which I will attempt to demonstrate in what follows when clues taken from within the book are combined with knowledge of psychopathology.
I believe Mr Jaggers has a separate coexisting disorder that relates to his forefinger biting.What is interesting is I think from the descriptions given within this book of fiction that the two disorders taken together would be a good convincing diagnostic fit for present day psychiatric diagnoses.
I think that Mr Jaggers has underlying Obssessive Compulsive Disorder ( OCD ) and I will show how he fits current diagnostic criteria for this conditon and furthermore his forefinger biting is a condition called Dermatodaxia by referrence to a case report on pub med which shows close correspondence to the description in the book .
First OCD :
We know from multiple instances that Mr. Jaggers has recourse to frequent handwashing ( preferably with scented soap ) as we know from one such example in Chapter 26:
“ My guardian was in his room, washing his hands with his scented soap, when I went into the office… "
“ I embrace this opportunity of remarking that he washed his clients off, as if he were a surgeon or a dentist. He had a closet in his room, fitted up for the purpose, which smelt of the scented soap like a perfumer’s shop. It had an unusually large jack-towel on a roller inside the door, and he would wash his hands, and wipe them and dry them all over this towel, whenever he came in from a police court or dismissed a client from his room.
Great expectations Chapter 18 , pp201-2
We can see that Jaggers is subject to compulsive handwashing rituals and is obsessessed by the thought of contamination by his criminal clients .
This would fit with an OCD diagnosis when taken together with the abscence of another disorder to explain his symptoms and in particular we know that he is not substance addicted (as far as we can tell from the novel) , he is if anything a high functioning workaholic and perfectionist ( seemingly always winning his cases and prepared to go to great lengths to do so ) .
Now his multiple episodes of forefinger biting exemplified by the episode at the Three Jolly Bargemen .This really does ressemble a disorder called dermatodaxia :
“ Dermatodaxia refers to skin biting in humans. Individuals who bite their skin may concurrently have other body-focused repetitive behavior-related conditions affecting their hair, nails, and/or skin. Similar to the man in this report, dermatodaxia is usually asymptomatic, unilateral, and affects a solitary site.”
The above case report is particualry telling in its close parallel to the account given in the book . It is worth noting that that dermatodaxia often coexists with OCD .
What is noteworthy is that Dickens appears to have provided a valid clinical description of an individuals symptoms to such full and close extent as to convincngly suggest current valid psychiatric diagnoses.