What does ELiza say that shocks the other guests at least some of them? How does Higgins explain ELiza's slang?

From the text:

LIZA [in the same tragic tone] But it's my belief they done the old woman in.

MRS. HIGGINS [puzzled] Done her in?

LIZA. Y-e-e-e-es, Lord love you! Why should she die of influenza? She come through diphtheria right enough the year before. I saw her with my own eyes. Fairly blue with it, she was. They all thought she was dead; but my father he kept ladling gin down her throat til she came to so sudden that she bit the bowl off the spoon.

MRS. EYNSFORD HILL [startled] Dear me!

LIZA [piling up the indictment] What call would a woman with that strength in her have to die of influenza? What become of her new straw hat that should have come to me? Somebody pinched it; and what I say is, them as pinched it done her in.

MRS. EYNSFORD HILL. What does doing her in mean?

HIGGINS [hastily] Oh, that's the new small talk. To do a person in means to kill them.