Since this word is made up of a single kanji, it probably uses the kun'yomi reading. You did not learn this reading, so here's a mnemonic to help you:
Snap all the bowstrings! Now none of the musicians can play their instruments. "You're too rude (つる)!" they yell, as they cry over the snapped bowstrings. They aren't wrong, that was just too rude of you.
The guitar strings are loose.