The phrase "didst top of head" indicates something that is being recalled from memory without prior preparation. Some synonyms for this phrase include offhand, without thinking, spontaneously, impromptu, and reflexively. These words imply that a person is recalling information quickly and without much consideration. Other synonyms might include unsystematically, instinctively, automatically, or intuitively. In any case, the phrase "didst top of head" suggests that the information being recalled may not be fully accurate or well thought-out, but is instead based on a quick recall from memory. As such, it can be a useful phrase for indicating a level of uncertainty or hesitation about information being provided.