"Firago" or "virago" is a word that is only really applicable to a woman; its orginal meaning is "a woman who is virile," who acts like a man. Maybe Sir Toby is playing with Sir Andrew's stupidity by referring to a supposedly dangerous man as a virago, but Viola truly is a virago, a woman who is acting like a man—"Cesario."
Anne Hathaway as Viola/Cesario