To wish someone Godspeed is to ask for God's blessings on his or her endeavor, most notably a long journey or a risky but potentially rewarding venture. Some view godspeed as a more reverent alternative to "good luck," which arguably introduces chance and randomness into the mix, not divine providence. One might wish Godspeed on a relative embarking on a long business trip overseas, for example.
The confusion over the meaning of Godspeed, which may also be rendered as god-speed or even goodspeed, lies in the definition of speed. The original meaning of the Old English word speed had nothing to do with velocity, but rather prosperity and good fortune. The addition of God to the concept of financial bounty may sound jarring at first, but the word Godspeed was an acknowledgment of God's generosity and blessing. Speed in that sense was the righteous acquisition of wealth and property through hard work and reverent behavior.
Some sources suggest that the word Godspeed may have been a corruption of "good speed," a wish for a speedy journey aided by favorable winds and sailing conditions. Under this theory, the expression good speed eventually became god-speed and ultimately Godspeed. The application of a Deity's name was simply a happy accident. Indeed, there are some recorded incidents of "good speed" being used as a blessing for ship-bound passengers.
The Middle English translators of the Holy Bible also used the term Godspeed in several passages, primarily in the Old Testament to indicate a God-inspired prosperity. This would suggest two separate evolutions of the expression, based on two different translations of the word speed. To wish someone Godspeed would be to wish him or her a prosperous journey or successful endeavor, while to wish someone goodspeed would suggest a swift and safe trip.
The expression Godspeed has largely fallen out of popular usage, and is generally listed as archaic in many dictionaries. Some still use it as a more theological blessing than a simple "good luck" or "bon voyage," although the original meaning has been forgotten in the mists of time.