What is most closely the meaning of formidable it is used in the following paragraphs?

Then Miss Hopley did a formidable thing. She stood up. Had she been standing when we entered she would have seemed tall. But rising from her chair she soared. And what sha carried up and up with her was a buxom superstructure, firm shoulders, a straight sharp nose, full cheeks slightly molded by a curve line along the nostrils, thin lips that moved like steel springs, and a high forehead topped by hair gathered in a bun. Miss Hopley was not a giant in body but when she mobilized it to a standing position she seemed a match for giants. I decided I liked her.