"It was turning out the dining-room done it." [closed]

“It was turning out the dining-room done it, if you ask me,” said Mrs. Sutton. “Now, don’t you overdo yourself, ma’am,” I says to her; but you know how she is, sir. She gets that restless, she can’t bear to be doing nothing."

Is the first sentence "It was turning out the dining-room done it," correct? It's in the novel Suspicion by Dorothy Sayers.

Something seems to be missing in this sentence and I don't understand the meaning of this sentence at all.

I would understand it in the following way:

What was the cause of her being unwell? It was (her) turning out the dining room, (that was what has) done it (read: her not being well for "it").

Or: (that has) done it.

I would not use the term incorrect English. Sayers renders the way simple uneducated people speak and that is interesting and attractive. If you render the language of a four-year-old child it doesn't make much sense to say it is incorrect English. That's the way little children speak.

In uneducated sub-standard speech a relative pronoun is often omitted even if it is a nominative. And the auxiliary verb is sometimes omitted, too.

Mrs. Mummery is tired.

Why is she tired, Mr. Mummery might wonder.

"Mrs. Mummery is tired because she works too hard. I warned her, but she insisted on turning out the dining room today" says the help, Mrs. Sutton.

"That's what did it. That's why she's resting."

(As I am an American, I have never turned out a dining room, but I have cleaned it and rearranged things in it. But I'm pretty sure I've not turned one out. To me, that's what you do with very, very, very bad house guests.)

It means that tidying the dining room, and perhaps discarding some of its contents, was the cause of the lady’s condition. The speech is in a non-standard dialect in which the past tense of ‘do’ is not ‘did’ but ‘done’.