A word you might say to someone setting out on a journey and I've always imagined it expressed hope for a swift journey. But TIL it has a deeper meaning, because speed is from English sped or speod, which meant 'success, good luck'. So it was wishing that God would grant them a successful journey, rather than a rapid one.

I think we also see this meaning of speed reflected in the traditional phrase Speed the plough or Go. speed the plough, an agrarian blessing, as Wikipedia puts it.

Ultimately speed derives from a Proto-Indo-European root *spe- and this is also the origin of Latin sperare, 'to hope', so speed is related to despair, desperado, desperate, Esperanto, sperate, a rare and archaic legal term referring to a debt which has a reason hope of being repaid — and possibly also prosper, although this is debated.