When I was a lad it was common practice (amongst the lads) to toss something (it could have been most anything, up to but not including an actual, live hand granade) at someone whilst screaming "Think Fast!!" -- the yell being intended to put one off ones game.

I don' think we were playing at Chaucer back in the 50s, nor did we give the matter of proper grammar much of a glimmer; but acting fast (or quickly) was certainly required.