Seriousness or earnest effort; putting full energy into something rather than doing it half-heartedly.
試験勉強に本腰を入れて取り組み始めた。
I started studying for exams in earnest.
プロジェクトも軌道に乗ったので、本腰を入れて開発を進める。
Since the project is on track, we'll proceed with development in earnest.
今までお試し程度だったが、彼は本腰を入れて生活を変えた。
Until now it was just a trial, but he got serious and changed his life.