Prior to 28 March 2006, the Black Watch was an infantry regiment – The Black Watch (Royal Highland Regiment) from 1931 to 2006, and The Black Watch (Royal Highlanders) from 1881 to 1931. Part of the Scottish Division, it was the senior regiment of Highlanders.
The source of the regiment's name is uncertain. In 1725, following the Jacobite rebellion of 1715, General George Wade was authorized by George II to form six 'watch' companies to patrol the Highlands of Scotland, three from Clan Campbell, one from Clan Fraser, one from Clan Munro and one from Clan Grant. These were to be "employed in disarming the Highlanders, preventing depredations, bringing criminals to justice, and hindering rebels and attainted persons from inhabiting that part of the kingdom." The force was known in Gaelic as Am Freiceadan Dubh 'the dark' or 'black watch'.
This epithet may have come from the uniform plaids of dark tartan with which the companies were provided. Other theories have been put forward, for instance that the name referred to the "black hearts" of the pro-government militia who had sided with the "enemies of true Highland spirit", or that it came from their original duty in policing the Highlands, namely preventing 'blackmail' (Highlanders demanding extortion payments to spare cattle herds).
However, these theories are without historical basis and do not stand up to scrutiny.
In 1739 the six Highland watch companies were augmented to ten and incorporated into the regular forces of the Crown as the Earl of Crawford's Regiment of Foot. The regiment was numbered the 43rd Regiment of Foot in 1747, changing to 42nd in 1749. In 1751 the regiment was titled '42nd (Highland) Regiment' and in 1758 was permitted the honor to add 'Royal' to its title. However, it continued to be known colloquially as the 'Black Watch'.
Source: Wikipedia
Image: badge and tartan