He was overconfident and thought he could pass the exam without studying.

overconfident 是由前缀 over-（表示"过度、超过"）和形容词 confident（自信的）组成的复合形容词。over- 表示程度超出正常范围，confident 表示"有信心的"。overconfident 合起来表示"过于自信的、自负的"。