"Girl Friday"

What does the phrase "Boy Friday" or "Girl Friday" mean exactly? Where did it come from? (Could it be from the character, Friday, in Robinson Crusoe?)

They mean an indispensable assistant: a cross between a secretary, a PA, a valet, a chauffeur, a maid-of-all work, etc.

Yes, they do come from Man Friday in Defoe's novel.

DFG