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недолго думая stresses
gen.	without pausing to think (Anglophile); after a moment's consideration (Andrey Truhachev); off-handedly (Andrey Truhachev); offhandedly (Andrey Truhachev); without hesitation; without a moment's hesitation; without thinking (Джозеф); at short notice (Ivan Pisarev); in a flash (Ivan Pisarev); in a New York minute (Ivan Pisarev); instantaneously (Ivan Pisarev); soon afterward (Ivan Pisarev); straightaway (Ivan Pisarev); straight off (Ivan Pisarev); this instant (Ivan Pisarev); tout de suite (Ivan Pisarev); without delay (Ivan Pisarev); in no time (Ivan Pisarev); at once (Ivan Pisarev); at the drop of a hat (Ivan Pisarev); quick as a wink (Ivan Pisarev); right away (Ivan Pisarev); without demur (Ivan Pisarev); with all speed (Ivan Pisarev); without a second thought (Ivan Pisarev); straight away (Ivan Pisarev); straight out (without thinking Andrey Truhachev); without a moment’s hesitation; without a moment's thought (Anglophile); without stopping to think (Anglophile); lightly (Andrey Truhachev); unthinkingly (Andrey Truhachev)
Gruzovik	without thinking twice
amer.	wasting no time (Val_Ships)
idiom.	on the spur of the moment (Andrey Truhachev); in a heartbeat (в русском эта фраза используется преимущественно в прош. вр. Баян)
inf.	head-first (without sufficient forethought Val_Ships)

недолго думая : 6 phrases in 3 subjects

General	2
Makarov	1
Proverb	3