To steel oneself or make up one's mind; to resolve to face a difficult or worst-case situation and accept it.
転職するために、腹をくくって退職届を出した。
I made up my mind to change jobs and handed in my resignation.
手術するならリスクもあると腹をくくって決めた。
I decided to have the surgery, accepting that there are risks.
彼は最後までやり抜くと腹をくくったようだ。
He seems to have resolved to see it through to the end.