To one's heart's content; with full force or as much as one wants; doing something vigorously or wholeheartedly.
休日は思いっきり寝てリフレッシュした。
I slept as much as I wanted on my day off and felt refreshed.
試合で思いっきり走って、最後まで粘った。
I ran with all my might in the match and hung on till the end.
ストレスが溜まっていたので、カラオケで思いっきり歌った。
I had been stressed, so I sang my heart out at karaoke.
子どもたちは公園で思いっきり遊んでいた。
The children were playing to their heart's content in the park.
Resolution or decisiveness — a noun meaning the quality of being determined or resolute.
転職には思いっきりが必要だと彼は言った。
He said that you need decisiveness when changing jobs.
彼女は昇進の誘いを断る思いっきりを見せた。
She showed the resolve to decline the promotion offer.