He asked me to marry him, and I said I had to think it over.

think 的意思是"思考、考虑"，指用头脑进行判断或做出决定的过程。 over 在这里是副词，表示"从头到尾、彻底地"的意思。 think it over 是一个固定短语，意思是"仔细考虑、认真思考"，强调对某件事进行全面而深入的思考。