In hindsight, the movie which pioneered the genre was 1972's Pink Flamingos with its infamous dog excrement eating scene, followed by 1974's sketch comedy sleeper The Groove Tube. The first movie to which the label "gross-out movie" was actually applied was 1978's National Lampoon's Animal House, which was a great box office success despite its limited production costs and thus started an industry trend. Characteristically for the genre, there are actually few gross (disgusting) scenes in it, but the humour is generally crass, fast and destructive.
Since the 1980s, gross-out films increased in number, and became almost the norm for US-American comedy films. Some films of this genre could be aimed at teen audiences (such as Porky's or American Pie), while others are targeted at somewhat more mature audiences (such as There's Something About Mary or Wedding Crashers).