What does God Speed mean to Christians ?

[Mouzafphaerre]

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Precisely "Allah'a emânet ol" (to whom that leaves or stays) or "Allah'a emânet" (for something that is cared/feared for).

Linguistics stuff below:

The second one includes an untold "olsun" or "ola" at the end, which is the third person imperative or optative of the verb "ol-" ("be") respectively. Therefore, the first statement is directed to the second person while the latter is concerned for an absent third person.
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[Shahed]

Brilliant. That's the perfect explanation. Thanks for that Mouz.
I thought it was Turkish

[Tribesman]

May the wind be at your back , another version for both land and sea .
God speed was also used for Pilgrims following pilgrimage routes , the roads and waystations were maintained by the church so god (through the church) could speed his worshipers on their journey .

[Shahed]

Wow, never knew there was so much behind it.