If the good of a thing consists in its proper operation or function, its final cause, then all we have to ask in order to begin to understand what constitutes modesty is: "What is the purpose of clothing?" What is its function? The obvious answer is to protect us from the elements. But once that is accomplished, can we adjust or modify our apparel in some way in order to serve a secondary purpose? Some of us said that clothing is for the sake of self-expression. That is true; who can deny it? So, what is it that a person tries to express in his choice of apparel? If we can answer that question, we are closer to solving this difficulty.
The school uniform expresses the fact that you are a student of this school. That identity, however, is temporary. The choice of clothing you would wear to a funeral would express something in you, i.e., your sense of grief. People often wear black, because black was at one time an unfashionable color. At a funeral, one does not want to appear to be concerned about looks; we want to express that we are not interested in ourselves at this time, but that our hearts "go out to" the one who has lost a loved one and who is grieving. Wearing bright and joyful colors to a funeral calls attention to you, they do not express the feeling of grief, and so they do not express a sense of solidarity with those who have lost a loved one. So we can argue convincingly that the "wrong" apparel manifests a lack of empathy. It fails to accurately express one's interior, or what one's interior ought to be (i.e., solidarity with the grieving).
Similarly, we dress for a party accordingly; we wouldn't wear a black suit and a serious demeanour. Moreover, the reason clergy wear black is that at one time, as we said, black was unfashionable, and clergy and religious (nuns, monks, etc.) saw dress as an expression of one's interior disposition (one's character). They chose to dress unfashionably in order to express that they are unconcerned about the goods of this world and that their lives are directed towards eternal goods (i.e., the kingdom of God). To be joyful and at the same time to dress unfashionably is to give genuine witness that one's joy does not come from the goods of this world, that is, from looking good.
But what about those who are not priests, monks, sisters, etc. The requirement is to dress modestly. But what does that mean? What is modest for one culture or time period is immodest for another. So how do we know? The same way we know the good of anything; we look to its final cause. The purpose of clothing is to express one's character, that is, who you are. Now, our fundamental moral obligation is to cultivate morally beautiful character (the kalon ). Thus, one should dress "beautifully", that is, in a way that expresses "beautiful character". That does not necessarily mean always dressing "to the hilt". Dressing simply may be an expression of one's character; dressing professionally may be an expression of one's character, etc.
Now, we are drawn to people of like character, and we are drawn to clothing or apparel that expresses who we are, or who we want to become. So, "Is this or that person dressing modestly?" Does this person dress himself or herself with reasonable restraint? The following questions may help to answer this question: When we look at this person, what do we see? Do we see a morally beautiful person? Or do we see too much? Does the clothing draw us towards this person as a person , that is, as a person of intelligence and moral beauty ? Or does the apparel draw us to this person as someone who can possibly satisfy a sexual appetite (lust) within us? If you are a woman, you can ask yourself: "Does this apparel accurately express who I am?" If you are a person who wants to arouse another or others sexually, then you will dress accordingly. The problem in this case, however, is your character. You are the kind of person who would like to be reduced to an object of sexual consumption, or the kind of person who has little regard for the moral integrity of others insofar as you intend to focus their gaze on you in a way that compromises their character. It may also be indicative of a lack of due self-respect. Why? Because you are more than that; but some women are so desperate to be loved that they will go to great lengths in order to be loved and desired.
Now some women are just not aware that what they are wearing is inappropriate (unbefitting a person of morally beautiful character), because they are unaware of how their apparel is affecting others, in particular those of the opposite sex. Women are not visually stimulated, but men are. Some women are unaware of this, so they are oblivious to the effects of their revealing clothing. They might also assume that "if everyone dresses this way, it must be okay".
Moreover, some men are not attracted to women of good character, that is, women who are intelligent, prudent, shrewd, and morally beautiful, because they know that they cannot sustain their facade for long, that these women will eventually see through them and thus reject them. They are drawn to women who are not so smart, not so emotionally healthy, but more needy and of lesser moral character. Such women are willing to do what it takes to be accepted; they are more open to being used. The woman who dresses in a way that appeals to that kind of man (character) opening up a path to trouble; she is attracting to herself the wrong kind of person.
The more we think about this, the more we should see that these questions are not easy to resolve. We should avoid the two easy alternatives to difficult moral questions: the extremes of absolutism and relativism. There is an objective standard, namely reason, but its application is not fixed and unchanging like a steel ruler. It is more like the measuring tape tailors use: it bends to various situations, but the standard remains universal (15 inches is 15 inches, even though every neck is somewhat different). Some instances of women's apparel are easily identifiable as immodest, others are barely immodest and barely modest, thus very difficult to determine; other instances are safely in the modest range, and others still are without much doubt excessively modest (they hide a woman's character by making her look either ugly or funny, or strange, slightly off, when in fact she isn't strange or off).
In the end, I don't really know for certain if a bikini is immodest. It says a lot that at one time, no one but strippers would model it. Cultural arrogance allows us to easily dismiss those of another period as sexually repressed; it is quite possible, however, that there is something wrong with us. I do lean heavily towards the old style swimwear, only because when looking at such women, they remain beautiful and personal. Many women on the typical beach these days appear desirable; very few strike me as beautiful and personal. This is not to deny that they are morally beautiful and personal; rather, their chosen apparel simply fails to reveal it.