adjective

spoken or done without preparation.

an extemporaneous speech

extempore,extemporary,impromptu,spontaneous,unscripted,ad lib,on-the-spot,improvised,improvisatory,unrehearsed,unplanned,unprepared,unarranged,unpremeditated,makeshift,thrown together,cobbled together,off-the-cuff,spur-of-the-moment,off the top of one’s head