Greasers was originally a job classification for folks who worked in mechanical maintenance trades like automotive, factory machinery, locomotives, or on board ships. They were typically covered in the pervasive grease that went along with their jobs.
Towards the beginning of the twentieth century, the term was also used as a rude derogatory label for those of Mexican ancestry. Because of poor access to clean water, migrant workers from Mexico sometimes had greasy hair.
In the post-WW2 years, the term greaser changed to describe disaffected youth who were from middle-class and lower families. Many worked on hot rods and motorcycles, and the culture developed around the use of working-class work wear like leather jackets and jeans. The young men slicked back their hair using grease, vaseline, or pomade like Brylcreem.
Famous greasers include Marlon Brando in "The Wild Bunch" and The Fonz from the retro 1970's television show "Happy Days". A whole musical was produced called Grease, and it was also a smash hit movie starring Olivia Newton-John and John Travolta.
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