undevised (?.)

• ad-lib
• caught napping
• caught off balance
• caught short
• extemporaneous
• extemporized
• haphazard
• hasty
• impromptu
• improvised
• inadvertent
• makeshift
• precipitate
• rough-and-ready
• snap
• surprised
• taken aback
• taken by surprise
• taken unawares
• tripped up
• unarranged
• unbegun
• unconcocted
• uncontrived
• undeliberated
• undesigned
• unhatched
• unintended
• unmade
• unmanufactured
• unorganized
• unplanned
• unpremeditated
• unprepared
• unprimed
• unpurposed
• unready
• unstudied
• unthought