The phrase "doth top of head" can be replaced by several synonyms, including "off the top of my head", "from memory", "without preparation", and "impromptu". These phrases all imply that the speaker is offering information or an opinion without taking the time to look it up or think it through thoroughly. "Off the top of my head" is perhaps the most commonly used of these synonyms, and is often used to convey a sense of spontaneity or informality. It can be a useful phrase in many contexts, from casual conversations to business meetings, where quick thinking and decisive action are required.