To be completely exhausted; to be worn out (to the point of having no energy left).
長時間の会議で皆が疲れきっていた。
Everyone was exhausted after the long meeting.
マラソンを走り終えて彼は完全に疲れきった。
He was completely worn out after finishing the marathon.
子育てで毎日疲れきっている母親も多い。
There are many mothers who are exhausted from raising children every day.