Gemara

50b: Rebbi was marrying off his son R. Shimon. He did not invite Bar Kapara (lest Bar Kapara make him laugh). Bar Kapara rebuked Rebbi for this; Rebbi invited him.

(Rebbi's afflictions used to atone for the world.) Any day that Rebbi would laugh, punishments would come to the world.

Rebbi (to Bar Kapara): Do not make me laugh, and I will give you 40 measures of wheat.

Bar Kapara said that he will get as much as he wants. He took a big basket, sealed it with tar and put it on his head. He asked Rebbi for the 40 measures of wheat 'that you owe me (for what I lent to you).' Rebbi laughed, and rebuked Bar Kapara for making him laugh.

Bar Kapara: I meant, the 40 measures that you owe (promised) to me.

Question (Bar Kapara): Why is homosexuality called To'evah?

Bar Kapara refuted every answer Rebbi gave. He asked Rebbi's wife to mix a drink for him, and offered to answer if Rebbi will dance. Rebbi danced.

Answer (Bar Kapara): TO'Eh Atah VAH (you stray in it).

Bar Kapara requested that Rebbi and his wife do so again before answering why bestiality is called Tevel, and why illicit Bi'ah is called Zimah?

Berachos 30b: Rabah saw that Abaye was overly jovial. He rebuked him: it says "v'Gilu bi'Re'adah"!

Abaye: I am wearing Tefilin (so all can see that I have accepted the authority of my Master over me).

R. Zeira similarly rebuked R. Yirmeyah, for it says "b'Chol Etzev Yihyeh Mosar." R. Yirmeyah answered 'I am wearing Tefilin.'

Berachos 31a (R. Yochanan): One may not fill his mouth with laughter in this world -- "Az Yemalei Sechok Pinu u'Leshonenu Rinah." Only then will laughter fill our mouths; i.e. when the nations will say (in the future) "Higdil Hash-m La'asos Im Eleh."

Reish Lakish never filled his mouth with laughter after he heard this from R. Yochanan.

Megilah 25b (Beraisa): We substitute less explicit words for blatantly derogatory words, e.g. "Yishkavenah" in place of "Yishgalenah"... "l'Motza'os" in place of "l'Machara'os";

R. Yehoshua ben Korchah says, we read l'MaCHaRa'os (as it is), for this disgraces idolatry (Beis ha'Ba'al was made into a Beis ha'Kisei. CHoR alludes to the anus.)

(Rav Nachman): Scoffing is permitted only regarding idolatry. We learn from "Kora Bel Kores Nevo...; Lo Yochlu Malet Masa" (the idols could not hold in their excrement until reaching the Beis ha'Kisei).

R. Yanai says, we learn from "L'Eglos Beis Aven... Ki Aval Alav Amo u'Chmarav Alav Yagilu Al Kevodo Ki Galah Mimenu..." (they mourned the loss of the idols over which the priests used to rejoice) - we read this 'Kevedo'.

(Rav Huna bar Mano'ach): One may tell an idolater to put his idol in his Shin Tov (buttocks, as in Chasufai Shes (Yeshayah 20:4)).

Rishonim

Rif and Rosh (Berachos 21a and 5:1): One may not fill his mouth with laughter in this world -- "Az Yemalei Sechok Pinu u'Lshonenu Rinah."

R. Yonah (DH Asur): Some say that the Isur is due to the Churban, but it is permitted when the Mikdash stands. This is wrong. If so, it should have said 'after Churban Beis ha'Mikdash...'! Rather, it says 'in this world...' Simchah accustoms a person to forget Mitzvos. After Hash-m will do miracles for us and save us, we will rejoice in order to reveal His wonders and mighty acts. The Nochrim will recognize what Hash-m did for us, and will be ashamed. Such Simchah in the Creator is a great Mitzvah to publicize the miracle. The Medrash (Shir ha'Shirim 1:4:3) expounds "Nagilah v'Nismechah Bo", i.e. we rejoice in Hash-m to publicize his awesome acts.

Prishah (OC 560 DH Schok): It seems that the Heter "Az Yemalei..." does not refer to the world to come, rather, to Yemos Moshi'ach or Bayis Sheni. The previous verse says "B'Shuv Hash-m Es Shuvas Tziyon Hayniu k'Cholmim." We expound that this refers to the 70 years of Galus Bavel. R. Yonah means that the Isur is as long as the Yetzer Ra overpowers us, for then Simchah causes one to forget Mitzvos. After the redemption it will be permitted to publicize the miracle. After the miracle is known, it will be forbidden again until Bitul of the Yetzer Ra in the world to come. The Levush explains according to R. Yonah that the Heter will be only in the world to come, when we will delight in the radiance of the Shechinah. This is wrong.

Rif and Rosh (Megilah 16b and 3:24): All scoffery is forbidden, except for ridiculing idolatry - "Kara Bel Kores Nevo...Lo Yachlu Malet Masa ...u'Chmarav Alav Yagilu Al Kevodo." We read this 'Keveido', i.e. its desire was removed and it does not help at all. One may tell an idolater to put his idol in his Shin Tov.

Rashi (Megilah 25b DH u'Chmarav): We read 'Keveido', (idols) laden with excrement.

Poskim

Shulchan Aruch (OC 560:5): One may not fill his mouth with laughter in this world.

Beis Yosef (DH v'Asur): The Tur and Ramban say that the Isur is only during exile, unlike R. Yonah.

Taz (7): Surely Simchah not of a Mitzvah is forbidden even when the Mikdash stands. Simchah of a Mitzvah is permitted when the Mikdash stands, e.g. Simchas Beis ha'Sho'evah or David's Simchah in front of the Aron. During Galus, Simchah has faded. One may not fill his mouth even with Simchah of Mitzvos such as a Nisu'in or Purim.

Kaf ha'Chayim (39): Some Chachamim explained that they were overly jovial because they were wearing Tefilin. The Zohar says that one should have great Simchah in Mitzvos. This is Simchah of the heart - "Pekudei Hash-m Yesharim Mesamchei Lev."

Kaf ha'Chayim (40): Tana d'Vei Eliyahu (13) says that excess Simchah brings anger to the world, and produce is reduced. Shevet Musar warns against Simchah in this world. Chivut ha'Kever (a harsh punishment after death, before Gehinom) is mostly due to such Simchah, i.e. not of Mitzvos.

Bedek ha'Bayis (Sof Siman 157): Filling one's mouth with laughter, sleeping more than a horse's sleep during the day, and having relations in years of famine are Isurim mid'Rabanan. There are no lashes or excommunication for them, but he is called 'one who transgresses Takanas Chachamim.'

Shulchan Aruch (YD 147:5): One may scoff about idolatry.

Rema: One may tell a Nochri 'may your god help you' or '...prosper your deeds.'

Levush (5): This is because it is permitted to mock idolatry.

Rebuttal (Taz 2 and Bach DH Kosav): It is forbidden to say so! The Nochri does not consider this mockery. The Yisrael encourages his idolatry! Darchei Moshe (1) brings this from his mistaken text of Perush ha'Mishnayos (Gitin Sof Perek 5). The Mishnah permits encouraging Nochrim working in Shemitah! The correct text of Perush ha'Mishnayos permits saying 'Elokim (not Elohecha) should help you.'

Acharonim

Maharsha (50b DH Yoma): Bar Kapara knew that Rebbi's afflictions atoned for the world. Nevertheless, he thought that he would merit the world to come by making him jovial, like it says in Ta'anis (21a).

Note: The Gemara said so about making sad people jovial, or jesting to quiet a quarrel. Neither of these applied to Rebbi at his son's Nisu'in!