Avodah Zarah 75b: According to the opinion that spoiled taste [coming from a pot used more than 24 hours ago] is permitted, then in what case did the Torah forbid cooking with the pots of non-Jews? Rav Chiya son of Rav Huna said: The Torah only forbade a pot that was used within the last day, for that is not spoiled taste. Why then is it not completely permitted to use such a pot after a day has passed? – It is a Rabbinic decree on treif pots after a day, lest one come to use a treif pot on the same day.
עבודה זרה עה ע”ב: ולמאן דאמר נותן טעם לפגם ־ מותר, גיעולי עובדי כוכבים דאסר רחמנא היכי משכחת לה? אמר רב חייא בריה דרב הונא: לא אסרה תורה אלא קדירה בת יומא, דלאו נותן טעם לפגם הוא. מכאן ואילך לישתריִ! גזירה קדירה שאינה בת יומא משום קדירה בת יומא.
A woman who was becoming frum, and only had a certain amount of money to spend, asked Rabbi Ahron Leib Shteinman, “Should I buy new modest clothing, or should I buy kosher dishes?” Rav Shteinman replied, “Buy the modest clothing, because the prohibition of using your old treif dishes is only Rabbinic, and it’s only a aveirah for yourself, but the aveirah of wearing immodest clothing is a Torah prohibition of not placing a stumbling block before the blind, and it involves sins for others as well.”
Source: Rabbi Ahron Shmuel Pessin
[What is the source for the idea that a woman wearing immodest clothing is “placing a stumbling block before the blind”? It may seem obvious, but one could argue as follows: The Gemara says in Bava Basra 57b that If a man walks down a path where he can see women washing clothing in the river, if there was another path he could have taken, he is wicked, but if there was no other path, he can’t help it and he is not held responsible for the sin. Still, it is praiseworthy to close his eyes. Thus it is possible that a woman could walk out in immodest clothing and cause no man to sin. All the men walking in the street with her might be on their way somewhere and have no other path to walk on. On the other hand, a man might have a different path available to him but still walk on her path, or he might gaze at her on purpose, which would be a sin no matter what.
This brings us to the question of whether “lifnei iver” – placing a stumbling block – applies only when one knows that the other person will commit the sin if he makes it available to him, or even if one doesn’t know. The classic case of “lifnei iver” is handing a cup of wine to a nazir or a limb of a live animal to a Ben Noach (Avodah Zara 6b). There the giver indeed knows that the taker intends to consume the item. However, the Gemara also says that one who smacks his grownup son is transgressing “lifnei iver” (Moed Katan 17a), although is not known whether he will hit back. Similarly, lending money without witnesses is “lifei iver” (Bava Metzia 75b) because the borrower may deny the loan, although it is far from certain that he will do so. So we can conclude that making a possible sin available also constitutes placing a stumbling block.
One might ask that the Gemara does permit selling wood to an idolatrous temple of fire, on the grounds that most wood is used for regular burning (Nedarim 62b). And the Mishnah says that although one may not sell a plow in Shmitah, one may sell a tool that can be used for either prohibited or permitted work (Shviis 5:6). The answer must be that where the item itself is definitely forbidden, and the doubt is only whether the person will commit the sin or resist his impulse, then giving it to him is placing a stumbling block. We are not allowed to test another person’s power to subdue his yetzer hara. But where the item itself can be used for a permitted purpose, perhaps the buyer is simply buying it for that permitted purpose, and he will not need to stuggle with his yetzer hara at all.
In our case, wearing immodest clothing would definitely be placing other people in a struggle with their yetzer hara, so it is “lifnei iver.” This is especially true if the woman in question lived in a neighborhood with non-religious Jews, who would certainly sin.
In fact, there is a Gemara in Taanis 24a that seems to say that immodest behavior is “lifnei iver”. We are told that Rabbi Yossi of Yukras had a beautiful daughter. One day he saw a man making a hole in the fence around his house. The man explained, “If I was not privileged to marry her, can’t I be privileged to see her?” Rabbi Yossi then said to his daughter, “My daughter, you are causing people to suffer. Go back to your dust, and let people not stumble because of you.” Rabbi Yossi of Yukras was criticized by Rabbi Yossi bar Avin for not having mercy on his own daughter. Still, she must have done something wrong, or else she would not have deserved any punishment at all. And the language “let people not stumble” indicates that her sin was “lifnei iver.”]