Meaning of Jo's dialog in Chapter 16 of Bleak House

Solution 1:

Bear in mind that neither the 'servant' nor Dickens' respectable middle-class readers were expected to understand this; it is the slang of the (illiterate) gutter-snipes. Knowing Dickens, I imagine it is an accurate rendering; but he carefully avoids making understanding necessary to the plot.

For what it's worth, I would say that fly survives in slang meaning 'knowing', so I'm fly would mean 'I understand'; fen is an adjective meaning marsh-related (there's a reference later to the 'mire') and larks means (via 'partying') affairs or business, so that fen larks means 'Follow my customs if you come with me'; and Stow hooking it, which he explains as meaning stow cutting away, means 'stop "recoiling from" me or pretending you're not with me'. But I'm not at all certain, and there is a distinct shortage round here of street-urchins who would interpret for a few pennies.

Solution 2:

"Marsh-related partying"? Nah.

Fen larks means "play fair" or "no cheating!" Stow hooking it/cutting away means "no running away!"

She asked him to keep his distance and not to look at her, he's telling her "okay, but no running off without paying me!"

Solution 3:

The Barnes & Noble Classics Edition, with notes by Tatiana Holway contains the following footnote in clarification of the first line spoken by Jo in this passage:

I understand. ... But no tricks, you know! Don't try running off!

For the second, it has the footnote:

Don't try running away.

Solution 4:

J.S. Farmer & W.E. Henley, in Slang and Its Analogues (1890–1904), cover most of the slang terms used by Jo in the line

"I'm fly," says Jo. "But fen larks, you know. Stow hooking it!"

Bleak House was published in 1852, and Farmer & Henley use the novel as a reference for several slang terms, so the authors were certainly interested in the particular meanings of Jo's various terms. Here are Farmer & Henley's relevant definitions:

FLY, ...Adj. (common.)—1.Knowing ; ARTFUL (q.v.) ; up to every move ; cute [in the sense of acute]. Also FLY TO, A-FLY, FLY TO THE GAME,and FLY TO WHAT'S WHAT. ... 2. (common).—Dextrous.

FEN, ... Verb (schoolboys').—(also FEND, FAIN, FAINITS, etc.). A term of warning, or of prohibiion : as to prevent any change in the existing conditions of a game ; e.g., at marbles, FEN-PLACINGS=no alteration in position of marbles is permissible ; FEN-CLEARANCES=removal of obstacles is forbidden. [FEND=M.E. defend in sense of 'to forbid.']

LARK, subs, (colloquial.)—1. See quot. 1811 {which reads as follows: "Lex. Bal., s.v. LARK. A piece of merriment. People playing together jocosely"}. [A corruption of M.E. lak, laik, from A.S. lac = game, sport; cogn. with Icl. leikr = game ; Sw. lek; Dan. leg; Goth laiks].

STOW, verb. (Old Cant).—1. To hold one's tongue; to keep quiet; to leave off

HOOK, ...TO TAKE 9or SLING) ONE's HOOK (or TO HOOK IT), verb. phr. (common.)—To decamp ; to run away.

So collectively the words mean essentially what Anon Y Mous and Phillip in Atlanta say in their answers:

I understand. But no games, and don't think of running way.

Dickens repeats the use of "hook it" in chapter 46 of Bleak House, where Jo tells Allan Woodcourt (a doctor) of a conversation Jo had with Inspector Bucket just before running away:

'Hook it! Nobody wants you here,' he ses. 'You hook it. You go and tramp,' he ses. 'You move on,' he says. 'Don't let me ever see you nowheres within forty mile of London, or you'll repent it.'