Synonyms of posthaste

1 Syllable
swift, soon, fleet, quick, fast, pdq
2 Syllables
fasted, promptly, breakneck, headlong, flat-out, speedy, quickest, fastest, swiftly, quicker, soonest, quickly, hasty, sooner, asap, pell-mell, rapid, chop-chop, fleetly, faster, pronto
3 Syllables
double-quick, speedily, directly, rapidly, apace, punctually, straightaway, double-time, screamin', hastily
4 Syllables
hypersonic, lickety-split, harefooted, expeditious
5 Syllables
incontinently
6 Syllables
precipitately
7 Syllables
unpremeditatedly