posthaste
[adverb]
Synonyms: fast, flat-out, fleetly, full tilt, hastily, lickety-split, quickly, rapidly, speedily, swiftly
[adjective]
Synonyms: fast, breakneck, expeditious, fleet, harefooted, hasty, quick, rapid, speedy, swift

[TahlilGaran] English Synonym Dictionary ▲

posthaste /ˌpəʊstˈheɪst/ US /ˌpoʊst-/
adverb OLD-FASHIONED FORMAL
as fast as possible:
They travelled posthaste to Rome to collect the award.

[TahlilGaran] Advanced Learner's Dictionary ▲