'Disguise' does not simply mean 'costume', though in this case she would certainly wear an old woman's clothes. disguise v. to modify the appearance or manner in order to conceal the identity of (oneself, someone, or something) (transitive) to misrepresent in order to obscure the actual nature or meaning n a mask, costume, or manner that disguises the act of disguising or the state of being disguised (Collins Dictionary)
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