Synonyms and related words :
ad-lib, be caught napping, be surprised, be taken unawares, be unprepared, cook up, dash off, do offhand, fake, go off half-cocked, have no plan, improvisate, improvise, jury-rig, knock off, lash up, make up, play by ear, scrap the plan, strike off, throw off, throw together, toss off, toss out, vamp, whip up, whomp up, wing it