He's such a dreamer, always fantasizing about becoming a famous singer instead of doing his homework.

dreamer 是由动词 dream（梦想）加上后缀 -er 构成的名词，表示"做...的人"。在这里指"经常做白日梦的人"或"空想家"，带有轻微贬义，形容那些沉溺于幻想而不切实际的人。