[frankvdwaal]

In German there's a similar word, "fahren", which means driving or traveling. In Dutch the word "varen" mean to sail or in an older sense of the word "to move".

I can only assume, but considering the Dutch "vaarwel" is so close to English, I'm going to guess it means "Go well" - or more poetically when speaking of one's path in life: may fate treat you well.

[vanviegen]

The Dutch word "welvarend" (literally "well-sailing") translates to "prosperous" in English. So "vaarwel" or "farewell" is kind of a medieval way of saying "live long and prosper". :-)

[eru]

There's also welfare and Wohlfahrt in German.