What are Pigs
Pigs are investors who assume high degrees of risk, or overlook risk entirely, with a singular focus on short-term profit. They make rash investment decisions or buy stocks without conducting enough or any research. As a result, pigs tend to lose money and potentially a lot of it -- hence the adage they get "slaughtered."
Bulls and bears have opposite investing styles, but each type of investor can make money over the long-term by investing according to their stated investment goals and strategy. Pigs, by contrast, eschew the buy-and-hold strategy and approach investing inconsistently.
The phrase, "Bulls make money, bears make money, pigs get slaughtered," is also the title of a 2002 book by Anthony M. Gallea containing general investing tips and advice.