Why answer with "which it is"? [duplicate]

I realise it's probably an outdated form, but this exchange (from Master and Commander) makes me wonder:

Captain: Killick! Killick there! What do you have for us tonight?

Cook: Which it's soused hog's face.

Captain: Eh?

Cook: Which it is soused hog's face!

Captain: My favourite. My favourite.

I really don't understand, from a grammar point of view, how those words go together as a (correct) sentence. Can someone please elucidate?

I immediately thought of Joe Gargery's contorted language in Dickens' Great Expectations; though to my surprise I find only one example of exactly this:

"Well!" said Miss Havisham. "And you have reared the boy, with the intention of taking him for your apprentice; is that so, Mr. Gargery?"

"You know, Pip," replied Joe, "as you and me were ever friends, and it were looked for'ard to betwixt us, as being calculated to lead to larks. Not but what, Pip, if you had ever made objections to the business—such as its being open to black and sut, or such-like—not but what they would have been attended to, don't you see?"

"Has the boy," said Miss Havisham, "ever made any objection? Does he like the trade?"

"Which it is well beknown to yourself, Pip," returned Joe, strengthening his former mixture of argumentation, confidence, and politeness, "that it were the wish of your own hart." (I saw the idea suddenly break upon him that he would adapt his epitaph to the occasion, before he went on to say) "And there weren't no objection on your part, and Pip it were the great wish of your hart!"

I take it to be reanalysing "which" in an exchange like:

"And this is the dinner?"

"... which is soused hog's face."

as though it is just a conjunction linking back to the previous sentence, rather than as a relative pronoun. But it is rather confused.