1. at a fast speed; rapidly: "Reg's illness progressed frighteningly quickly"Similarfastswiftlyrapidlyspeedilyat high speedwith all speedat (full) speedat the speed of lightat full tiltas fast as one's legs can carry oneat a gallopbrisklyat the doublepost-hastewith all possible hastelike a whirlwindlike an arrow from a bowat breakneck speedexpeditiouslymadlyhotfootwith dispatchinformal:double quickin double quick timep.d.q. (pretty damn quick)nippilylike (greased) lightninghell for leatherat warp speedlike madlike crazylike blazeslike the windlike a bomblike nobody's businesslike a scalded catlike the deucea mile a minutelike a bat out of helllike a bullet out of a gunlike the clappersat a rate of knotslike billy-olickety-splitliterary:apaceOppositeslowly