The phrase "didst the top of your head" is a unique and somewhat archaic way of referring to something that someone knows or remembers off the top of their head. While this phrase might not be commonly used today, there are plenty of synonyms that convey the same idea. Some common options include "offhand," "immediately," "without hesitation," "spontaneously," "instinctively," and "unprompted." These phrases suggest a person is responding quickly and without much thought, relying on their own knowledge or experience to answer a question or provide information. Overall, while "didst the top of your head" may not be in current everyday language, there are plenty of other expressions that can fill the same semantic space.