weak knees

"I tell you—he'll kill you," repeated Carley, backing away until her weak knees came against the couch.

The difficulty, such as it was, was soon over, and the rest of the descent was only a trial for weak knees.

I've read he was a mean-lookin' cuss, with weak knees; but he was all horse: and ain't Waster got bad knees?

But one day when Pelle came stealing home in this way, Lasse received him with a radiant face and weak knees.

We know of a shoer in Worcester County, Massachusetts, who has a wide local reputation for "doctoring" weak knees.