The story
Univerbation of God speed, elliptical for God speed you, where speed carries the archaic sense “help, further, cause to prosper”. Compare God bless, God damn.
Etymology from Wiktionary, CC BY-SA 4.0.
godspeed
God·speedToday it means: Used, particularly at a moment of departure, to convey a wish that an individual’s forthcoming actions yield favorable outcomes; typically addressed to someone embarking on a journey or undertaking a challenging endeavor. Full definition →