I think it's more about how she acts than what she wears. For the most part, if she's wearing a skirt, dress, or shorts that her ass is falling out of or a top that her boobs are literatley ready to bounce right out of, then that's trashy and makes her look very insecure and desperately attention seeking. That's not appropriate for children to see and it looks bad in front of parents. Like if a guy were to surprise his mom with a visit from his girlfriend and she had that on, it would most likely leave a bad impression
Its not what you wear, its what your attitude is like when you wear it, if you go with your mood and don't get to bitchy in general, then I don't think you will look slutty, but if you can be bitchy and dress that way, you well could come across as slutty,x
Sexy is natural. It's what you end up with when you wear clothes that look good on your body shape and shoe off a little bit but without looking like you're trying to get attention. Skanky is when you do so intentionally in a way that makes you stick out from the crowd.
Trashy shows too much at once and just looks cheap, like your clothes are poor quality and taste.
For example, some cleavage (as long as more than half your boob is covered) is okay but in that case don't wear a skirt that is very short, and if it is mid thigh I wouldn't show too much cleavage, maybe only a little bit.
If you are wearing a really short skirt or short shorts, don't show cleavage and wear sleeves.
Belly shirts always look trashy in my opinion.
Or heels that are just too high and bright looking
Or if there is a combination of very tight, a lot of leg and cleavage at once, that is probably trashy.
It also depends on your body. It kind of sucks but certain things definitely look trashier if you have big boobs and/or a big butt. Tight jeans or short dresses/short skirts for example. With a big butt, they all look trashier.
Sexy is showing a little bit but not all, leaving some to the imagination. Maybe a skirt that is fitted around your butt but not very short, and no cleavage showing with it.
Skanky would be the skirt that is short, tight, and paired with a shirt where your t*ts are hanging out.