This is a single kanji with しい on the end, meaning it's an adjective. What's the adjective form of long time? Well, it's long ago, or can mean for a long time.

Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:

Long ago, in a galaxy far away, a bunch of prequel movies were ruined by a single character. "Heesa (ひさ) the best character!" you hear. Oh no. Heesa here. Get out before Jar Jar Binks gets you! Keep him in the past!