This person is very sharp and shrewd. The expression refers to the fact that he or she doesn’t stand still long enough for flies to land. A late nineteenth-century Americanism, it appeared in Eugene Field’s poem “Jest ’fore Christmas” (1892): “Most all the time, the whole year round, there ain’t no flies on me.” It also was in the title of a Salvation Army song (1900), “There Are No Flies on Jesus” (“There may be flies on you and me, / But there are no flies on Jesus”). More recently, Robert Barnard used it (Death and the Chaste Apprentice, 1989): “‘Oh, I’ve ideas all right. There’s no flies on me, you know. I’ll get to the bottom of it.’”