The Black Hackle originated as a Scottish tradition to wear a black feather in your hat to signify you have an ongoing quarrel with someone.

It was adopted by several British Army

British Army

The British Army is the land warfare branch of Her Majesty's Armed Forces in the United Kingdom. It came into being with the unification of the Kingdom of England and Scotland into the Kingdom of Great Britain in 1707. The new British Army incorporated Regiments that had already existed in England...

 military units at various times, including No.9 and No.11 Commando

British Commandos

The British Commandos were formed during the Second World War in June 1940, following a request from the British Prime Minister, Winston Churchill, for a force that could carry out raids against German-occupied Europe...

 (Scottish

Scotland

Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

) during the Second World War. Also adopted by the Royal Ulster Rifles

Royal Ulster Rifles

The Royal Ulster Rifles was a British Army infantry regiment. It saw service in the Second Boer War, Great War, the Second World War and the Korean War, before being amalgamated into the Royal Irish Rangers in 1968.-History:...

, later amalgamated with other regiments into the Royal Irish Rangers

Royal Irish Rangers

The Royal Irish Rangers was a regular infantry regiment of the British Army.-Creation:...

.