It seems that this would be forbidden, but only Rabbinically. The Gemara (Nazir 59a) records a dispute among the Tana'im as to whether the prohibition for a man to shave his underarm and pubic hair - which is a practice of women - is from the Torah or Rabbinic. The Rambam (Hilchos Avodah Zarah, 12:9) rules like the lenient opinion. The Beis Yosef (YD 182) points out an apparent contradiction in the rulings of the Rambam. To wit, the Rambam also rules like the opinion of R' Eliezer ben Yaakov that all Tikunei Ishah - which would seemingly include shaving body hair - is forbidden by the Torah. The Beis Yosef resolves the contradiction by saying that even though R' Eliezer ben Yaakov holds that wearing any element of women's clothing is forbidden - even if the man does not look like a woman and could not easily mingle with women - nevertheless it must be something external and visible like a dress; something that could facilitate mingling with women. But something hidden from sight like shaving body hair would only be forbidden Rabbinically. Therefore, there is no contradiction with his first ruling that shaving body hair is only Rabbinic.

Other Rishonim (most noteably the SMA"G), however, rule like the strict opinion that shaving body hair is forbidden by the Torah and that this is also the opinion of R' Eliezer ben Yaakov. According to this view, wearing a women's undergarment would also probably be forbidden by the Torah.

It seems from the analysis of the Beis Yosef that the dispute between between the Rambam and the SMA"G is how to understand R' Eliezer ben Yaakov's interpretation of the word "To'eivah" (abomination) in the verse dealing with Beged Isha (Dvarim 22:5). According to the Rambam, "To'eivah" does not refer to the act itself of dressing up like a woman but rather to the end result of dressing like a woman (i.e. that it could lead to the abomination of forbidden relations between men and women). Therefore, only something that could lead to the To'eivah - that is, something visible - is forbidden. According to the SMA"G, the "To'eivah" is the act itself of wearing women's clothing. Therefore, even if it is hidden it is forbidden.

(One thing, however, that is agreed upon by all the Poskim and that is that the Halachah follows the opinion of R' Eliezer ben Yaakov (d'Mishnaso Kav v'Naki) who says quite clearly in the Beraisa that the prohibition of Beged Ishah does not depend on the intention of the man to mingle with women.)

The Shulchan Aruch (YD 182:5) rules like the opinion of the Rambam and the Biur haGRA explains this opinion in a similar fashion to the Beis Yosef. Therefore it would seem that wearing a women's undergarment would only be forbidden Rabbinically.

It should be noted that both the Ba"Ch and the Ta"Z (loc. cit.) hold that there is no prohibition - even Rabbinic - if a man wears a women's garment - even an exteranl garment - for utilitarian purposes, for example, to protect himself from the cold. According to this view, if a man wears a woman's undergarment because he has nothing else and he needs it to absorb sweat, or for whatever other reason undergarments are worn, it might be permissible.

Kol Tuv,

Yonasan Sigler

This is not a Psak Halachah