We don't know for sure if Andrew Mitchell called police officers plebs.
According to reports in The Sun, the 56-year-old chief whip ranted "learn your f***ing place" at officers outside the Downing St gates, as he attempted to cycle down the street, calling them "plebs" and "morons". Mitchell has apologised for his behaviour, but denies he used such language.
But how insulting is it to call someone a "pleb"?
According to the Oxford Dictionary, the word pleb originates from Ancient Rome.
Plebs weren't slaves, but they weren't aristocrats, the members of the "lower social classes." The term literally means "of the common people."
Privately-educated Andrew Mitchell could have learnt the term in school. As Guido Fawkes pointed out on Twitter: "Early public schools like Rugby would enrol pupils as "plebeians" as opposed to sons of gentry and aristocrats."