Every now and then you still hear people use the term "she's my girl Friday." If the classic movie that inspired the saying had stuck with its source material, they'd have to say "my boy Friday," which doesn't have quite the same ring.
His Girl Friday, starring Cary Grant, was based on the play The Front Page. Director Howard Hawks, contemplating an adaptation, wanted to hear if the dialogue was still as fresh and funny as he remembered, but he didn't have two men around to read the parts—so he read one while his secretary read the other. Hawks loved the male/female dynamic and made a gender switch.
Though the role of Hildy Johnson was an incredibly strong woman who had just as much status and respect as her male costars, nobody wanted the role. Katharine Hepburn, Irene Dunne, Claudette Colbert, Jean Arthur, Margaret Sullavan and Ginger Rogers all said "no" before Rosalind Russell won the part—which launched her to stardom and is now widely regarded as one of the best female comedic roles of all time.