Completely exhausted; dog-tired — describing someone limp, weak, or worn out (mimetic word).
徹夜で仕事をして、朝にはもうヘロヘロだった。
I worked all night and was totally exhausted by morning.
マラソンの後で足がヘロヘロになった。
After the marathon my legs were completely exhausted.
子どもたちを一日中連れ回して、夜はヘロヘロで寝てしまった。
I spent the whole day taking the kids around and fell asleep utterly worn out at night.
ヘロヘロになりながらも最後まで作業を終えた。
Even though I was exhausted, I finished the work to the end.