The "eephus" pitch was first thrown by Rip Sewell back in the 1940s; he threw it maybe 5-10 times per game.
In the 1970s Steve Hamilton threw it once in a while, you can see an example here:
Dave LaRoche made it part of his repertoire in the early 1980s, at the end of his career; when he threw it, they called it "LaLob".
The way those guys threw it was nothing special, they just lobbed the ball high in the air with regular backspin and tried to get it to drop somewhere near the strike zone.
Orlando "El Duque" Hernandez throws a very slow, high curveball that is similar but not quite the same thing.