Denim shirts were connected to the American working class and the West Frontier in the early 1900s. They stood for tenacity, fortitude, and the spirit of the pioneer.
Denim was taken up by the counterculture movements of the 1960s and 1970s as a symbol of defiance against conventional fashion and social mores. Jeans and a denim shirt became a standard for people who opposed traditional values and promoted social change.