A portion of chapter 16 of Dickens' Bleak House is shown below.
Jo attends closely while the words are being spoken; [...] and nods his ragged head.
"I'm fly," says Jo. "But fen larks, you know. Stow hooking it!"
"What does the horrible creature mean?" exclaims the servant, recoiling from him.
"Stow cutting away, you know!" says Jo.
I don't understand either the gist or the details of what Jo says in the above. I've assumed I'm fly means he understands or is agreeable or something; that fen is a noun or verb and larks is vice versa; that stow means stop or quit; that hooking it and cutting away mean getting away. However, I don't understand what he is trying to communicate. Any ideas on Jo's meaning? (I presume it's all explained in annotated versions of the book, but I don't have an annotated copy; and the numerous occurrences of this passage on the web that I've looked at are given as examples of difficult-to-understand parts of Dickens, without explanation of the meaning.)