Before this century, I never heard 'modest' being used to describe clothing. I would have recognised what a nun meant, for instance, if she spoke of 'modest clothing' but I'd have found it quite amusing.
Modest means moderate, not bragging or showing off. For some reason, 'modest' has now been replaced by 'humble' on social media. (Humble really means something far more self-effacing than modest.) This leaves poor old 'modest' with nothing to do but act as a fashion diktat.
So ... moderate, unostentatious clothing. Those parameters vary by situation, because clothing is primarily a social instrument. All you really need is a blanket, or nothing if it's hot. We wear clothes to meet our culture's standards of decency: some cultures say no flesh may be exposed, some say cover the genitals, some say full nudity's okay. Beyond that, we wear clothes to advertise where we belong, our social or professional rank, our beliefs or clan/tribe, our personalities and so on.
'Moderate' dress for a gala ball looks completely different from the 'moderate' outfit you might wear to an office interview. Some social occasions actually require ostentatious dress, so a modest outfit would single you out as (immodestly) making a point. But, should someone admire your ultra-flashy attire, your reply would be modest if you have good manners.
I'm no scholar of medieval Arabic, but I strongly doubt that the word translated as 'modest' actually means modest. It probably means something like 'meeting the rules of decency as understood in 7th century Saudi Arabian society'.
As a concept to be applied across time and through many cultures, a modest outfit should be a middle-of-the-road effort to blend in with the prevailing norms: not trying to show off or make a point. If being worn by a young woman at a party in a night club, it would likely be tight or revealing. If you want 'modest' to mean something closer to medieval Saudi Arabia, you really should choose a different word.
What is it that you're trying to tell your daughter, @coffeegirl73? To wear more boring clothes? To look like she's not proud of herself? To cover 20% more of her flesh than other women her age? If you manage to put your finger on it, you'll be a lot closer to understanding why you feel it's better to dress that way.