This is a single kanji with しい on the end, meaning it's an adjective. What's the adjective form of mourning? It's unpleasant or disgusting. Nobody said mourning was fun.
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:
Your mourning is especially unpleasant because it's your pet eagle (い) you're mourning for. Your parents wouldn't let you bury him so you're just staring at his week old, dead eagle body. It's super disgusting, actually.
Please erase this disgusting memory somehow.