Gemara

58b (Rav): One may shave his entire torso with a razor.

Question (Beraisa #1): A man who shaves his underarms or by the genitals is lashed.

Answer: One is lashed for shaving with a razor. Rav permits with scissors.

Objection: Rav said 'with a razor!'

Answer: He meant cutting very close with a scissors, like with a razor.

Version #1 (R. Yochanan): One who shaves his underarms or Ervah (near the genitals) is lashed.

Question (Beraisa #2): Removing (this) hair is forbidden only mid'Rabanan.

Answer: R. Yochanan meant that he is lashed mid'Rabanan.

Version #2 (R. Yochanan): One who shaves his underarms or Ervah is lashed due to "v'Lo Yilbash Gever Simlas Ishah."

Question (Beraisa #2): It is forbidden only mid'Rabanan.

Answer: R. Yochanan holds like the following Tana:

(Beraisa #3): One who shaves his underarms or Ervah transgresses "v'Lo Yilbash Gever Simlas Ishah." (end of Version #2)

Question: Beraisa #2 forbids only mid'Rabanan. How does that Tana expound "v'Lo Yilbash Gever Simlas Ishah"?

Answer (Beraisa) Suggestion: Perhaps "Lo Yihyeh Kli Gever Al Ishah (v'Lo Yilbash Gever...)" forbids a woman to wear a man's garment, and vice-versa!

Rejection: The Torah calls this "To'evah" (but this is not an abomination).

Answer #1: A man may not wear a woman's garment and sit among women, or vice-versa.

Answer #2 (R. Eliezer ben Yakov): A woman may not bear weapons in war, and a man may not adorn himself like women do.

(Rav Nachman): A Nazir may shave the underarms and Ervah.

The Halachah does not follow Rav Nachman.

Question (Rabanan): We saw that R. Yochanan does not have hair there!

Answer (Rav Shemen bar Aba): It fell out due to old age.

R. Ami sentenced a man to be lashed, and saw that his underarms were not shaven. R. Ami deduced that he is a Chacham, and commanded not to lash him.

Question (Rav): May one shave there if the hair will grow to annoy him? This is not for adornment!

Answer (R. Chiya): No. When the hair gets too long, it falls out.

Question (Rav): May one scratch there so that the hair will fall out?

Answer (R. Chiya): No, but one may scratch through a garment.

Yevamos 103a (Mishnah): Chalitzah above the knee is Pasul. (It is not called 'Regel'.)

Question: "Lo Osah Raglav (Mefivoshes did not remove the hair near the Ervah)."

Rishonim

Rif and Rosh (Makos 4a and 3:4): Rav permitted shaving the entire torso with a razor. We asked from the Beraisa that lashes one who shaves his underarms or Ervah. We answered that Rav permits other limbs. We asked from another Beraisa that forbids mid'Rabanan, and answered that Rav permits cutting very close with a scissors, like with a razor.

Rosh: One may cut hair with a scissors from the rest of the body, other than the underarms and Ervah. These are forbidden even with a scissors.

Rambam (Hilchos Avodah Zarah 12:9): The Torah permits a man to remove hair from other parts of the body (than the beard or sideburns), e.g. the underarms and Ervah. Mid'Rabanan it is forbidden and one is lashed for it. This is in a place where only women remove it, for the he adorns himself like a woman. In a place where men remove it, he is not lashed. One may remove all other hair with a scissors.

Gra (YD 182:3): The Rambam holds that Chachamim never permit. In some places, people do not need this.

Question (Rosh Yevamos 12:8): Our Gemara connotes that one may shave the Ervah. In Nazir (58b) Rav permits cutting there with a scissors (very close) like a razor. However, at the end of the Sugya Rav asked about shaving there if the hair grows to annoy him. R. Chiya said that the hair never gets too long. This implies that he does not permit cutting it at all. It would notbe astounding to say that Rav did not accept the answer!

Answer (R. Tam, cited in Rosh): Rav asked about shaving with a razor. He said 'it gets long' to show that this is done to avoid pain, so it is unlike Simlas Ishah, which is for beauty. R. Chiya answered that it does not grow too long, so there is no pain. BaHaG's text (like that of the Rif) connotes that Rav permits other limbs with a scissors like a razor, but not the Ervah and underarms. We asked from the Beraisa that forbids the underarms and Ervah, and answered that Rav permits other limbs. We asked from another Beraisa, and answered that Rav permits cutting very close with a scissors, like with a razor.

Rosh: BaHaG's text does not argue with ours about the Halachah. When we answered that Rav permits very close with a scissors, we retracted from the distinction between the underarms or Ervah and other limbs. However, if so we must say that other limbs are forbidden with a razor, and permitted with a scissors like with a razor. From the case with R. Ami, it seems that people were stringent not to do so. The Ge'onim permit shaving the underarms and Ervah only if one shaves the entire body from head to toe. Then, it is disgraceful. Surely, it is for healing, not for beauty.

Ran (Avodah Zarah 9b DH Ro'eh): Where men remove hair from the underarms and Ervah, it is permitted. Only Chaverim refrain. In R. Ami's place, most men did so. This is how he knew that the man was a Chaver.

Tosfos (Nazir 59a DH Chazina): The Gemara asked why R. Yochanan has no hair in the underarms, and answered that it fell out due to old age. It did not answer that he cut it with a scissors. He must forbid this mid'Rabanan.

Poskim

Shulchan Aruch (YD 182:1): One who removed hair from the underarms or Ervah even with scissors, if he cuts very close like with a razor, used to be lashed mid'Rabanan.

Beis Yosef (DH u'Mah she'Chosav): What the Rosh wrote in Makos overrides what he wrote in Yevamos, for the primary Sugya was in Makos. Also in Yevamos he said that the practice was to forbid the Ervah and underarms. If it is not very close, even these are permitted, for this is unlike Tikunei Ishah.

R. Yonah (Igeres ha'Teshuvah, cited in Beis Yosef 181 DH v'Chosav Adoni): Chachamim forbade removing hair with a razor anywhere on the body.

Gra (2): Most rule like the first version of R. Yochanan (who forbids mid'Rabanan), because the Beraisa supports him. BaHaG rules like Version #2, because the Halachah always follows R. Eliezer ben Yakov.

Shulchan Aruch (ibid.): This is in a place where only women remove it, for this is like Tikunei Ishah. In a place where men remove it, he is not lashed.

Rema: (In such a place) it is permitted l'Chatchilah. In all places Chachamim refrain from this.