This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of the fat? It's the same: fat.

This is the kun'yomi reading of the kanji (because it's a single kanji, all alone with hiragana attached). You learned that reading with the kanji, so you should be good to go. But here's a mnemonic just in case:

The fat cat loves to curl up on the futon (ふと) all day.