What is meant when somebody says "God's speed"?

It is usually said to someone who is going somewhere or going to do something potentially dangerous. It means to hurry home and be safe. Although, when looking at various phrase finders, I came across an obscure mention of this and the Bible. According to what I read, in biblical times, God's speed meant the opposite. You were in fact asking the person to join the other side so to speak. How much merit that has, I have no idea. It is certainly not what is meant when said today.

I think it is probably short for 'May God speed you on your journey', meaning a wish that God should help you on your way.

First you have to define the words you are using. In this case two words: God and Speed.
God- the universe.
Speed- Relative speed in which the universe expands.

If god/universe is expanding, then it is doing it at a speed that is correct. We cannot call it too fast or too slow. It is God's speed. God is omnipotent- knows all, is all, etc.

Thus, God's speed is the correct speed at which to expand/travel/journey/move about space.

So when we say to our departing friend, "God's speed," we mean to tell them that God will dictate the speed at which we all move about. We mean to remind our friend that we must be mindful that our existance, past/present/future, is completely known in every aspect by the supreme being/god/universe.

"God's Speed" is old English for " Good Luck"

God·speed - noun

Definition of GODSPEED
: a prosperous journey : success

Origin of GODSPEED
Middle English god speid, from the phrase God spede you
God prosper you

First Known Use: 15th century

http://www.merriam-webster.com/dictionary/godspeed

An expression of good will when addressing someone, typically someone about to go on a journey or a daring endeavor.
http://en.wiktionary.org/wiki/Godspeed