the best one can do; one's best effort
with all one's might; to the best of one's ability; as hard as possible
試験に精一杯取り組んだ。
I gave my all to the exam.
今できる精一杯をやり遂げた。
I did the very best I could at this moment.
毎日精一杯働いてきたことを誇りに思う。
I am proud of working as hard as I could every day.