Koichi frowned at the DDOS attack and banged his fist on the table.
This is a single kanji with an い on the end, meaning you know it's probably an adjective. The adjective form of bitter is bitter, or more specifically, a bitter flavor.
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: She gives you some food, and it's terribly bitter. You look at the lady who gave you this bitter food. You say, "She boooo" as in, she's no good, and she serves bitter food.