A philosophical or general outlook that expects the worst outcome. Often contrasted with 楽観主義 (optimism).
His pessimism is affecting the people around him.
When you fall into pessimism, you can't get anything done.
Compound of 悲観 (pessimistic view) + 主義 (principle, -ism). 悲観 is itself a Sino-Japanese word composed of 悲 (sad, mourn) and 観 (view, outlook).