Marrow is usually a food, either the fatty whitish stuff in bones or a vegetable like a long pumpkin. But that's an obvious nonsense!
I've seen it used as a synonym for friend or companion; my dictionary (Collins - very good!) suggests it might have a Norse origin. Margr apparently means "friendly" in Icelandic! And as a dialect word in English, it's associated with the north-east of England (Durham, Northumberland) and up into Scotland, where you'd expect Norse influence, also meaning "friend" or "fellow-worker". So, even if the mar- element does go back to margr or marrow, the name would still mean "famous friend" rather than "famous vegetable" etc ... the medieval Norse wouldn't have known or cared about vegetable marrows anyway!
Hope that helps.