Mutually helping one another. A 1906 dictionary of Latin quotations lists manus manam lavat, ascribed to Seneca the Younger (5 b.c.–65 a.d.), and the current phrase may indeed be a translation derived from this source. In Deborah Eisenberg’s “A Cautionary Tale,” the term is used with regard to a character paying more to sublet an apartment than the original renter pays. “‘One hand washes the other,’ Marcia had remarked cheerfully when she explained this to Patty.” Another version of this cliché is “You scratch my back and I’ll scratch yours.” Novelist Lee Child had it in 61 Hours (2010): “He wanted combination radiation shelters and living accommodations built. He talked to the Air Force. He scratched their backs, they scratched his.” See also quid pro quo.