Oversized dress shirt, preferably thin linen, preferably white - put it back to front and button one button halfway up your back (could strategically hide bra strap) so you have a mainly open back (hot)
If the front is too loose / gives a little bit too much straightjacket vibes grab a couple safety pins and drape the fabric how you want, then pin on the inside of the shirt - this can be a bit finicky, but overall worth it for the end effect
Goes with anything, but personally I think it is perfect answer to jeans and a nice top - I've been wearing this every time I go out for drinks lately and I've been stopped by a woman on the street walking past me in a smoking area, and a waiter at a restaurant - both said they'd never seen anything like it before - also so cool for the summertime!
For some reason this is the only picture I have where you literally cannot see the shirt but I recommend this STRONGLY
