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Also, gal Friday. An efficient and faithful female assistant, as in I'll have my girl Friday get the papers together. The expression plays on man Friday, a name for a devoted male servant or assistant. The name Friday comes from Daniel Defoe's Robinson Crusoe, whose shipwrecked hero named the young native who became his faithful companion for the day of the week when he found him. In the mid1900s Friday was applied to a male servant and then a woman secretary or clerk who works for a man. The expression girl Friday gained currency through a motion picture starring Cary Grant and Rosalind Russell, His Girl Friday (1940). Today it tends to be considered condescending and, applied to a woman, sexist.
American Heritage Idioms