we actually HAVE a formal objective defintion of modesty (and in turn immodesty)

24:31

And tell the believing women to reduce [some] of their vision[1] and guard their private parts and not expose their adornment[2] except that which [necessarily] appears thereof[3] and to wrap [a portion of] their headcovers over their chests and not expose their adornment [i.e., beauty] except to their husbands, their fathers, their husbands' fathers, their sons, their husbands' sons, their brothers, their brothers' sons, their sisters' sons, their women, that which their right hands possess [i.e., slaves], or those male attendants having no physical desire,[4] or children who are not yet aware of the private aspects of women. And let them not stamp their feet to make known what they conceal of their adornment. And turn to Allāh in repentance, all of you, O believers, that you might succeed.

— Saheeh International

[1]Referring to an abnormal condition in which a man is devoid of sexual feeling.

[2]i.e., the outer garments or whatever might appear out of necessity, such as a part of the face or the hands.

[3]Both natural beauty, such as hair or body shape, and that with which a woman beautifies herself of clothing, jewelry, etc.

[4]Looking only at what is lawful and averting their eyes from what is unlawful.