The phrase "doth off the top of your head" is an idiomatic expression that means to mention or think of something without prior preparation or research. Some synonyms for this phrase include "off the cuff," "impromptu," "spontaneous," "unscripted," "unrehearsed," and "unplanned." These words all convey the idea of acting or speaking without prior planning or preparation. Other similar phrases that can be used interchangeably with "doth off the top of your head" include "on the spot," "without thinking," and "from memory." All of these words and phrases are useful when describing an action or response that is generated spontaneously.