1) I will leave aside for the moment the question of whether there is an Isur of going naked in the street, but there is another very important issue also involved here. This is the lack of self-respect that a person shows if he does such a thing, and the effects that this will have on his personality and general behavior.

2) We see this clearly from the Rambam, Hilchot Edus 11:5, who writes that the "Bezuyin" (shameful people) are invalid as witnesses, mid'Rabanan. An example of this are those people "who walk naked in the marketplace when they do their filthy work there." The Rambam writes that the reason these individuals are invalid as witnesses is because they do not care about their own embarrassment and shame. As a result of this attitude, these sort of people are not particular about whether or not they tell the truth in a court of law.

3) This passsage in the Rambam is recorded as law in the Shulchan Aruch (Choshen Mishpat 34:18, which deals with the Halachos concerning which people are invalid as witnesses), and the Vilna Gaon (Bi'ur ha'Gra #40) cites the Gemara here in Yevamos 63b as the source for the Rambam's ruling.

4) Accordingly, the sort of person who is Mehalech b'Shuk Arum is not believed in a court of law because he has cheapened himself so much that he no longer has any self-respect.

5) I am not sure what you mean when you say that we learned in Dafyomi that Mehalech b'Shuk is Mutar. However, the Gemara in Berachos 19b that states that if somebody is in the street and he discovers that he is wearing Sha'atnez, he must take it off immediately. The Gemara says there that even though this is extremely embarrassing, nevertheless the Chilul Hash-m involved in the fact that the wearer of the clothes is transgressing a Mitzvah of the Torah is even greater than the lack of honor that he brings upon himself by removing the clothes. However, I would say that, on the contrary, we see from that Gemara that while it is very bad to be Mehalech b'Shuk Arum, it is even worse to wear Sha'atnez.

(A way of approaching the question of whether there is an Isur on Mehalech b'Shuk Arum would be to discuss whether there is a Torah prohibition against a person putting himself to shame. Perhaps we will come back to this later.)

Kol Tuv,

Dovid Bloom

In this reply I will attempt to prove, b'Siyata d'Shmaya, that there is a Torah prohibition against going naked in the street.

1) I will start with the opinon of the S'mak, the Sefer Mitzvos Katan (written by one of the major Rishonim, Rabeinu Yitzchak of Courville, 1210-80). This is cited by the Mishnah Berurah, in the Bi'ur Halachah 3:1 (DH Yehei). The S'mak(Mitzvos Aseh #57) writes that Tzeni'us -- modesty -- is one of the 613 Mitzvos of the Torah. Therefore, I assert that soembody who is Mehalech Arum b'Shuk certainly violates the Mitzvah of modesty and transgresses a Mitzvah of the Torah.

2) I assert further that there is an additional prohibition involved here, namely the prohibiton against putting oneself to shame, which, according to most authorities, is a Torah prohibition.

This is a large topic, and I will have to be fairly concise on this. I will start with the Shulchan Aruch (Choshen Mishpat 420:31) who writes that a person is not allowed to injure himself. Now a similar question arises: Do we say that in the same way that a person is not allowed to inflict physical damage upon himself, he is not allowed to embarrass himself?

This question depends on the Sugya in Bava Kama 91b. We will look only at the conclusions from that Sugya. There is the opinion of the Me'iri (91b, DH Zeh), who writes that while there is a Torah prohibition to injure oneself, on the other hand if a person is not particular about his own honor, he is allowed to put himself to shame.

However, the majority of Poskim disagree. The Yam Shel Shlomo there (8:59, based on Tosfos, DH Ela Tana'ei) writes that the Halachic conclusion is that it is forbidden both to injure oneself and to embarrass oneself. See also Shulchan Aruch ha'Rav, Choshen Mishpat, Hilchos Nizkei Guf v'Nefesh #4, who writes that a person does not possess the rights over his body to permit him to shame himself. The Pischei Choshen, Hilchos Nezikin 2:2 (by Rav Yakov Blau zt'l, the Rav of Sanhedria, Yerusahayim), writes that it appears that the majority of authorities maintain that a person is not allowed to shame himself.

3) Returning to our Gemara in Yevamos 63b, I suggest another reason for why the Gemara says that this is "Meshukatz u'Metu'av." This is because the Gemara is discussing Bnei Noach (the people from Barbaria, etc.) who do this. They have only seven Mitzvos, and it follows that they do not have the Mitzvah of Tzeni'us. However, the Bnei Noach do have to fulfill the Mitzvos of the Torah that are logical. This is discussed at length by Rav Nisim Ga'on, in his introduction to Shas, printed in most Gemaras at the beginning of Maseches Berachos.

4) This, then, is the reason for why the Gemara says that Mehalech Arum b'Shuk is Meshukatz u'Metu'av. It is saying that there is a prohibition also for the people of Barbaria, etc., who are forbidden from doing this sort of thing only if it is logical to say they should not do so.

Kol Tuv,

Dovid Bloom

David, I sent your question to a Gadol and he replied that it is not written that there is a prohibition against Mehalech b'Shuk Arum, but a person who do this is "Meshuga." That is, no one in his right mind would do this in the middle of the street.

However, we can now understand why the person who finds Sha'atnez in his suit is required to take it off wherever he is. This is because there are certain extreme circumstances where one has to behave in a way that would otherwise appear to be very strange.

The source for this is the Mishnah in Maseches Eduyos 5:6 where Akavya said: "I would rather be called a fool for the rest of my life and not be considered as wicked for one moment before G-d." Accordingly, in order to avoid wearing an item of clothing which is forbidden by the Torah, it is worth suffering embarrassment.

Kol Tuv,

Dovid Bloom