This is a jukugo word that uses the on'yomi readings of the kanji. You haven't learned this reading for 糸 yet, so here's a mnemonic to help you:

Who only uses silk thread in their clothes? Sheep (し)! Sheep can't wear wool thread, oh no, what are you, some kind of monster?! Sheep all wear clothes made from silk threads while they're waiting for their wool to grow back.