Non-player character. Normally a completely reasonable term from video games where, depending on the game, most characters you encounter might not be players.

Then it started being used by the alt-right to refer to people who disagree with them and therefore aren't people who actually think for themselves but rather following a script, so using it to describe real people is kinda gross. The reasoning being "they believe what everyone else does so they're sheeple/NPCs". Because only controversial beliefs are an indication of being capable of thought.