MCP is a short, rude name made from the first letters of male chauvinist pig. It is an insult for a man who believes men are better than women and who holds tight to old rules: men lead, women serve, and that is that. People used it a lot in the 1970s, when many women were pushing back against those ideas. It is informal, sharp, and dated. Younger speakers may not use the letters at all; they are more likely to say sexist, or just describe what the man said or did. If you call someone an MCP, you are not being polite, and you are not making a kind joke. Though actually, the same three letters are used for other things in science and in computers, so this insult meaning only fits when the talk is about people and gender, not about machines.