He was pessimistic about the exam results and thought he would definitely fail.

pessimistic 是形容词，由名词 pessimism（悲观主义）派生而来。pessim- 这个词根来自拉丁语，意思是"坏的、糟糕的"，-istic 是形容词后缀，表示"具有...特性的"。pessimistic 合起来表示"悲观的、消极的"。