What's quinsy?

Quinsy is an old-fashioned word for tonsillitis — an acute inflammation of the tonsils. In Willa Cather's My Ántonia, Jim gets sick after going on a sled ride with Ántonia and her sister Yulka:

Yulka forgot to give me back my comforter, and I had to drive home directly against the wind. The next day I came down with an attack of quinsy, which kept me in the house for nearly two weeks.