gets her knife into someone

[Tinkerbell]

"After a while she seemed to make up her mind that there was nothing she could do about it. He'd go his way, she'd go hers. He wanted a son, of course. And there wasn't another child after Miss Theodosia. So Master Hadrian came to live here. But she, her ladyship, seemed to be more of a tartar every day; and once she gets her knife into someone . . . she's going to use it."
I said: "I should get out, I suppose."

Could you explain me pls that red part?

 

[Tdol]

When she starts to dislike someone, she will attack/hurt/criticise them at every opportunity.

 

[Tinkerbell]

Thank you :-D