In the seventies, a scandal occurred which involved Republican agents breaking into the Democratic headquarters of the Watergate hotel. After that, many major scandals had '-gate' appended to the name. 'Plebgate' refers to a small scandal involving Andrew Mitchell, a conservative member of the British Parliament, who was very rude to police who questioned him, calling them 'plebs' (lower class). The scandal was then 'pleb' + '-gate' = plebgate.