WordNet sense 1 (with little or no preparation or forethought):

ad-lib, extemporaneous, extempore, impromptu, off-the-cuff, offhand, unrehearsed, extemporary, offhand

WordNet sense 2 (without preparation; not prepared for):

unprepared

WordNet sense 3 (without previous thought or preparation):

offhandedly

WordNet sense 4 (marked by lack of attention or consideration or forethought or thoroughness; not careful):

careless