Hippie fashion, for iconic as it is, is incredibly hard to pin down. There were no major designers, and it wasn't appearing on the pages of Vogue or Harper's Bazaar. Extant hippie gear from the 1960s is incredibly rare. There's very little of it in museums, and when it appears for sale, it's often at a very high price.
I've been agonizing for weeks about how to define it.
Fashion is, as always, a reflection of societal values, and this is especially true for hippie fashion. At its center was rejection, as loudly and colorfully as possible, of the neat, tidy, and heavily gendered aesthetic of the 1950s.
This is why men with long hair was such an essential part of the hippie look. In the conservative American mindset, a man having short hair was a part of his proud, rigid masculinity, and anything longer than a standard military crew cut was both effeminate and unkempt.
Hippies were anti-materialists, hence the fact that they didn't have any major designers (although many designers took inspiration from hippies later on). Valuing hand-crafts, a lot of clothing was hand-made, or came from small artisan boutiques. This is where "vintage clothing" started being a thing, as they recycled clothes from the 1930s and 40s to fit their style.
Much inspiration came from Eastern cultures, especially those along the so-called "Hippie Trail" that ran from Europe, Turkey, through the middle east, to Afghanistan and Nepal, and then to Thailand.
Because they valued nature and natural things, they wore natural fibers and natural materials, often including buckskin and leather. Combining the love of the natural with the idea of free love, nudity was a big thing
And, of course, underlying all of this, was psychedelics. It all combined to make the hippie aesthetic.
The truth is, it's not the clothes, it's how you wear it. A lot of hippie style was just jeans and t-shirts worn with long hair and a disdain for The Man.
Hippie fashion is extremely fun and worth taking a good look at.
Check out this exhibit that was put on by San Francisco's Bellevue Art Musum