What derasha is Tosfos referring to? The Sefas Emes explained that we see from the sugya in Yevamos 76-77 that there was tremendous debate whether David haMelech could be appointed king or whether the fact that he was a descendent of Rus, an Amonis, disqualified him. It was testimony that the beis din of Shmuel had ruled that only Amoni, males of the nation of Amon, but not Amonis, females of that nation, were disqualified, that settled the issue. As a result of that ruling Shaul eventually lost the kingship and David was appointed in his place. Shmuel haNavi wanted confirmation that he was correct in making that derasha and acting to support David.
(According to this interpretation, the halacha of “Amoni – v’lo Amonis” is based on a derasha. The Brisker Rav reads the conclusion of the sugya that the din was a halacha l’Moshe m’Sinai. Something to discuss another time maybe).
If I were giving a women’s shiur this chag, this issue would be a tempting choice to speak about. The nation of Amon is excluded from intermarrying with Bnei Yisrael for failing to extend food and drink to Bnei Yisrael when they passed through their territory. Doeg ha’Adomi argued that this criticism applied equally to the men of Amon as well as the women and both should be barred. The gemara (76b) asks: but women normally do not go out to greet strange men – tzniyus!? The gemara answers that the Amonite men could have gone to greet the men of Bnei Yisrael and the Amonite women could have gone out to the women. We could have put up a big mechitza! If the women failed to go out, argued Doeg, they are equally to blame.
This question was the show stopper that would have disqualified David haMelech if not for the fact that someone remembered that Shmuel’s beis din had already paskened the issue otherwise. However you understand why Shmuel’s beis din was the final word in the matter (halacha l’Moshe m’Simai or an irrefutable derasha), the gemara grants the point. But what of Doeg’s tremendous kashe – how do you respond to his argument? The gemara answers: “Kol kvuda bas melech pnima.”
One way to read the gemara’s conclusion is that mechitza or no mechitza, it’s still unreasonable to expect women to go out and greet strangers. If this is correct, the whole shakla v'terya can be reduced to simply a matter of metziyus, a debate about the facts on the ground: is the expectation that the Amonite women should go out and bring food to their Israelite sisters reasonable or not? Originally the gemara thought Doeg had a great kashe and the expectation was not unreasonable, kah mashma lan that it was.
I prefer to avoid making a debate about an issue of metziyus. Instead, I think the gemara’s shakla v’terya gets to the root of what modesty is all about. There are two elements to modesty: 1) avoiding promiscuous behavior, especially intermingling of the sexes; 2) behaving with restraint and reserve. The Midrash (BaMidbar Rabbah 1:3) tells us that before there was an Ohel Moed, G-d spoke to Moshe from a burning bush in Midyan, G-d spoke to Moshe in Egypt, G-d spoke to Moshe from Har Sinai. However, now that there was an Ohel Moed G-d spoke only privately from that tent to fulfill the ideal of "hatzne’ah leches". Obviously we are not speaking about the intermingling of the sexes here. When the Midrash speaks of hatzne’ah leches, it means comportment, a certain type of behavior, a “madreiga pnimis,” as the Maharal describes it. See this post for more. So often we get caught up in issues of skirt and sleeve length that we forget all about this very important second element.
Doeg thought that given the need to do chessed, it is enough if the Amonite women were sensitive to that first element of modesty, avoiding intermingling of the sexes, but they should have put aside that second element in order to help Bnei Yisrael. So long as that mechitza was in place, there was no excuse for their inaction. The gemara in the end rebuts this argument. “Kol kvudah bas melech pnima” means that this second aspect of modesty, modesty of character, is primary. If the Amonite women chose to modestly stay indoors rather than go out and greet strangers, their behavior cannot be condemned, even if it meant the sacrifice of an opportunity to do chessed.
Perhaps this gemara alerts us to another possible reason for the reading of Megilas Rus on Shavuos. Chazal (see Rashi Shmos 34:3) tell us the fanfare and public spectacle involved in the giving of the first luchos led to their being shattered, which teaches us “ain lecha yafah min ha’tzeniyus,” there is nothing nicer than modesty. On the first day of Shavuos we relive the lightning and thunder of Sinai, the public display of awesomeness. The second day of Shavuos reminds us that as great as that experience was, there is yet a greater value – the value of modesty.