expression, godan verb (-su)
smooth out wrinkles; straighten out; relax
Literally to smooth out wrinkles from fabric or paper, and figuratively to relax a tense face or mood. Often used in the phrase 眉の皺を伸ばす (to unknit one's brows).
アイロンでシャツの皺を伸ばす。
I smooth out the wrinkles in the shirt with an iron.
彼の冗談を聞いて、彼女は
Hearing his joke, she relaxed her furrowed brow.