With the rise of the counter-culture and experimental film in the 1950s and 1960s, there were plenty of bizarre movies, but in the1970s, we start to see a plethora of films that descend into absolute insanity.
It's impossible to talk about weird movies of this era without bringing up "The King of Bad Taste" himself, John Waters. He eventually settled down and became more mainstream, but his films from the 70s were absolutely nuts. The most well-known is easily the transgressive classic Pink Flamingos from 1972, which starred the drag queen known as Divine and was made for only $10,000.
Pink Flamingos featured over the top acting that was far from naturalistic and was full of extremely offensive content, such as a man with a sausage tied to his penis, Divine giving unsimulated oral sex to an actor that portrays her son, and the also unsimulated ending which consisted of her eating actual dog poop. This is the only non-pornographic work I'm aware of that featured an actor going to such lengths.
There's plenty of other stuff that's just weird without being gross or disturbing, like the so-called egg lady and characters licking each other's feet. Waters' film was so far out there that it was banned in Australia as well as parts of Canada and Norway.
There have been imitators, but no one has made a film quite like Pink Flamingos since. Along with a couple other films I'll mention, it was crucial in the popularity of the midnight movie phenomenon that formed in the 1970s, where theaters started playing cult films late at night.
Waters' second most famous movie of the 70s was Female Trouble, also starring Divine and many other cast members from Pink Flamingos. It doesn't quite reach the depths of depravity of that film, but it's still very odd and includes a scene where two characters have sex and are both played by Divine, one female, and one male.
It wasn't just low budget movies that were strange in the 1970s, but expensive, mainstream films as well. The most obvious example is the self-indulgent science fiction film Zardoz, from acclaimed director John Boorman, who had already garnered an Oscar nomination for directing Deliverance.
Zardoz lets you know right from the start how weird it's going to be. After an intro featuring a disembodied human face on a black background, the first scene has a giant floating stone head announcing the "gun is good" and the "penis is evil". Then a bunch of guns spill out of its mouth. Boorman thought this scene would make it easier for audiences to understand, but he later admitted that this didn't quite work.
Zardoz starred Sean Connery, who was a huge name at the time having just come off portraying James Bond multiple times. He spends the film in a ridiculous outfit that consists of thigh-high boots and red shorts that resemble a diaper.
This film had a decently sized budget at over $1.5 million and barely made that back at the box office. It unsurprisingly didn't get a good critical reception and has gotten somewhat of a cult following over the years.