What is the source for dressing in fancy dress on Purim?

Question

I have searched in several sefarim for information about why we dress in fancy dress on Purim but, yet to no avail! Mishna Brurah, chaiyai Adam and the Kitzer all say to wear Shabbos clothes; they don't mention fancy dress. Where does this minhag come from? How comes it's become so prominent? What is the reason for it?

Answer 1

Ohr Someyach answers:

Dressing up on Purim is a long-standing custom; the earliest record is found in the writings of the Mahari Mintz , a late 15th century Italian Rabbi and scholar. He writes that on Purim it is permitted for a man to dress up as a woman and vice versa, and it is clearly implied that this is a custom that is well established in his day.

Chabad answers

There are several reasons given for the age-old custom to dress up in costumes(1) on Purim. Here are some of them:

1. In contrast to the overt miracles of the holidays of Passover,Chanukah and other Jewish holidays, the miracle of the holiday of Purim was disguised in natural events. [snip]

The custom of wearing costumes on Purim is an allusion to the nature of the Purim miracle, where the details of the story are really miracles hidden within natural events.(2)

2. The Talmud writes that just as the Jews at the time pretended to be serving other gods, G‑d pretended that He was going to destroy the Jewish nation, and in the end He did not.(3) Rabbi Tzvi Elimelech Shapiro (1783–1841), known as the Bnei Yissaschar, writes that this is the reason we pretend to be someone else on Purim, since both the Jews’ and G‑d’s actions were masked by other intentions.(4)

3. We dress differently on Purim to minimize the embarrassment of the poor who go around collecting charity on this day—a day when we give charity to everyone who outstretches their hand.(5)

4. To commemorate the dressing up of Mordechai in King Ahasuerus’s royal garments in the story of Purim.(6)

SOURCES

1. See Shulchan Aruch, Orach Chaim 697:8 (but actually it's 696:8).

2. Minhagei Kol Aryeh. See Rabbi Moses Hagiz (18th century), Eileh Hamitzvot, p. 293.

3. Talmud, Megillah 12a.

4. Bnei Yissaschar, vol. 2, in the chapters on Adar.

5. Minhagei Kol Yaakov.

6. Rabbi Shlomo Danah, Shalmei Todah (Inyanim Nifradim), p. 30.

Answer 2

The only "source" that I know for this custom is the Rema, Orach Chayim 696:8. He notes the phenomenon of wearing masks (פרצופים) on Purim, and the custom of men dressing up like women and women dressing up like men. His source for the foregoing is a teshuva by the Mahari Minz §17 (R' Yehuda Minz, 15th c.), which I haven't checked. Since the Rema notes that this is already a custom, his comment to that effect is hardly a source for it.

Here it is in his own language, followed by my translation (I have translated פרצופים as "costumes", rather than "masks", because of the context):

מה שנהגו ללבוש פרצופים בפורים, וגבר לובש שמלת אשה ואשה כלי-גבר, אין איסור בדבר מאחר שאין מכוונים אלא לשמחה בעלמא, וכן בלבישת כלאים דרבנן. ויש אומרים דאסור, אבל המנהג כסברא הראשונה

As for the custom of wearing costumes on Purim, with a man dressing in women's clothing and a woman wearing the attire of a man, there is nothing forbidden in this since one's only intention is general rejoicing - and so too with the wearing of rabbinically prohibited shaatnez. There are those who say that it is forbidden, but the custom is in accordance with the former opinion.