The boy's lumpish response made it clear he didn't grasp the lesson even after three explanations.

lumpish 是由名词 lump（块状物）加上形容词后缀 -ish 构成的形容词。lump 本意是"块状物"，引申为"笨重的、不灵活的"；-ish 表示"具有...性质的"。lumpish 合起来表示"笨拙的、迟钝的"，形容人反应慢或动作不灵活。