The math problem the teacher gave was a real stumper, nobody in the class could solve it.

stumper 是由动词 stump（难住、使困惑）加后缀 -er 构成的名词，表示"令人困惑的事物"或"难题"。stump 原指树桩，引申为"阻碍、难住"的意思，-er 后缀在这里表示"造成某种效果的事物"。