expression, godan verb (-ku)
feel relieved; forget one's troubles; find peace of mind
Idiomatic expression literally meaning 'to open one's eyebrows', used when worry or tension disappears and one feels at ease.
試験が終わって、やっと眉を開くことができた。
After the exam was over, I was finally able to feel relieved.
彼の無事を知らされて、母は眉を開いた。
Upon being informed of his safety, my mother's worries melted away.