The phrase "doth off top of your head" could be replaced with an array of synonyms. Some possible options include saying something "off the cuff," "on the fly," "impromptu," or "spontaneously." Similarly, one might "wing it," "ad-lib," or "freestyle" when speaking or presenting without prior preparation. Alternatively, you could describe the act of speaking without notes or a script as "speaking extemporaneously," "improvising," or "rambling." No matter which synonym you choose, the main idea remains the same: you are speaking without prior planning or preparation, relying solely on your own thoughts, ideas, and knowledge.