An assistant who is capable of many different types of tasks. I like to have my son be my man Friday during "take your child to work" day. He's a better worker than some of my employees!
Trusted assistant. This term comes from Daniel Defoe’s Robinson Crusoe (1719), in which Crusoe found a young savage on a Friday, and this man became his faithful servant and companion on the desert island. “I take my man Friday with me,” said Crusoe. Some mid-twentieth-century advertising pundit invented “girl Friday”—or gal Friday—to describe the female clerk-of-all-work, presumably on the assumption that it lent some glamour to a low-level, poorly paid position. It caught on mainly through being used as the title of a 1940 motion picture starring Cary Grant and Rosalind Russell, His Girl Friday. In the 1970s, when affirmative action came to the American labor market, the term fell into disrepute.