"Cut adrift" is also used metaphorically. Literally, as Daniel mentioned, it would be cutting the rope that holds a boat at the dock and letting it float away. If you say a person has been cut adrift, I would understand it to mean that she has been disconnected from her metaphorical anchor and is wandering aimlessly.
As to "military missionary," could you share the context? It might be as simple as Daniel's explanation, or it may have a different connotation.
Good luck with your studies!