Nowadays the expression associated with the activity of going from door to door on Halloween evening is usually trick or treating, but a traditional Scottish and northern English word for this activity is guising, and those who go guising are called guisers, defined in the OED as masqueraders or mummers.
The original meaning of the noun guise was manner, method or way. Another definition given in the OED is "style or fashion of attire or personal adornment" and this sense is seen in the expression in the guise of. The word guise came into English from French, but it is ultimately of Germanic origin and is cognate with the English word wise, meaning manner or mode, a word that is now obsolete except in compounds such as leastwise, likewise and otherwise.
The original meaning of the verb to disguise was "To alter the guise or fashion of dress and appearance of (any one); esp. to dress in a fashion different from what has been customary or considered appropriate to position, etc.; to dress up fantastically or ostentatiously" (OED). As the OED says, the leading sense of the verb these days is "To change the guise, or dress and personal appearance, of (any one) so as to conceal identity; to conceal the identity of by dressing as some one or in a particular garb". Disguise comes from the Old French desguisier or deguisier. The prefix des/de was altered to dis- in English due to Latin influence. The original function of the French prefix de- or des- was to undo or reverse the action of the verb -- the historical journey of the verbs disarm, disband and discolour is similar to that of disguise.