expression, godan verb (-su)
stretch one's arms; elongate one's arms
Rare, literary expression literally meaning 'to stretch out one's monkey arms', implying long, extended arms. Used in descriptive or figurative contexts.
彼は猿臂を伸ばすようにして遠くの物を取った。
He stretched out his long arms as if he were a monkey and grabbed something far away.
Common expression for 'stretch one's arms'; 猿臂を伸ばす is a rare, more vivid variant emphasizing length like a monkey's arm.
From 猿臂 (long arm, literally 'monkey arm') + を + 伸ばす (to stretch). The exact origin is uncertain, but it is a rare, literary phrase.