feeble or lacking strength

कमज़ोर

English Usage: The weak were left behind in the competition.

Hindi Usage: कमज़ोर प्रतियोगिता में पीछे रह गए।

having weak or unstable knees

कमज़ोर घुटने वाला

English Usage: After the long hike, she felt weak-kneed.

Hindi Usage: लंबी चढ़ाई के बाद, उसे कमज़ोर घुटने जैसा महसूस हुआ।