A belittling or uncomplimentary name given to someone who is not necessarily a communist but is understanding of communism and communists, and this person may support socialism. For some people Communism is associated with the color red, and pink, a lighter shade of red, could stand for the lighter shade, form, or type of communism, being socialism.
Dan said to Dave "Yeah, I don't have a problem with Stalin, I'd have supported him." "WHAT!?!?!?!" said Dave, "I'm all for being controlled by a totalitarian government, and having no economical classes, you know, everyone being equal?", "Dude, FUCK that shit and FUCK you,I'm gonna start callin you Pinko." said Dave. "What's wrong with communism?", said Dan, "Shut the hell up Pinko!", said Dave.