1 quickly, speedily, swiftly, rapidly, at once, immediately, instantaneously, promptly, without delay, right away, straight away, post-haste, hurriedly, directly, suddenly, in haste, precipitately, on the spur of the moment, in a flash or wink, before you can say 'Jack Robinson', Colloq pronto, like a shot, like greased lightning, US lickety-split, Slang p.d.q. (= 'pretty damned quick'):
She left hastily when I asked for the return of the loan.
2 impetuously, impulsively, rashly, recklessly, unthinkingly, thoughtlessly, heedlessly, incautiously:
This is a trick question, so don't answer hastily.