To have one's knees wobble or almost give out after long walking or strenuous exertion; knees trembling from fatigue.
山登りの翌朝は膝が笑って、階段を降りるのが大変だった。
The morning after the hike my knees were wobbling and it was hard to go down the stairs.
大会で長時間走ったら膝が笑って最後はペースを落とした。
After running for a long time in the race my knees started to wobble, and I slowed down at the end.
重い荷物を持って歩きすぎると膝が笑うから気をつけてね。
Be careful—if you carry heavy luggage and walk too much, your knees will start to wobble.