flipping someone the bird: showing someone your middle finger; an insult in many Western cultures
This term originated with a slightly different wording; originally “flipping someone’s bird”. In the medieval ages it was common for kings and lords to have pet birds (1) in their homes. These birds were quite rare and expensive, thus they were a sign of wealth; they were trained to sit on a horizontal metal rod for visitors to see. It became an extreme show of disrespect to literally “flip” someone’s bird: striking it with a middle finger so that it spun around and hung upside down (2). Eventually simply flicking the air with one’s middle finger was enough to suggest this disrespectful act and insult/threaten another lord. It is believed that the term was reworded over time – as actual bird flipping became rarer – and later generations copied the gesture without knowing its origin. They assumed that the middle finger was ‘the bird’ in question.
1 – Of course not just any bird would do, they specifically kept a rare breed of owl. It was originally an acronym that meant: only with lords. Today the term has been generalized to all members of the genus.
2 – Owls, like all nocturnal animals, fall asleep immediately when they find themselves upside down. An example of this can be seen in caves full of upside down sleeping bats.