The phrase "do top your head" means to give an answer or suggestion without having much thought or consideration. Synonyms for this phrase include impulsive, reflexive, offhand, and spontaneous. It suggests an answer given without much thought or preparation, often based on instinct or intuition. Other synonyms for this phrase are hasty, abrupt, and sudden. These words imply that the answer given was quick and not thoroughly thought out, which could result in errors or inaccuracies. Overall, it is important to think carefully before giving an answer, as it can have significant consequences and affect the way others view you.