Wikipedia:
The Black Watch (Royal Highland Regiment) was a line infantry regiment of the British Army, in existence from 1881 (as the Royal Highland Regiment (Black Watch)) until 2006. The regiment was created under the Childers Reforms of 1881, as the Black Watch (Royal Highlanders), by the amalgamation of the 42nd (Royal Highland) Regiment of Foot and the 73rd (Perthshire) Regiment of Foot.
As part of the Scottish Division, it was the senior regiment of the Highland Brigade. The regiment's name came from the dark tartan that they wore and from its role to "watch" the Highlands. The 'Black Watch' was originally just a nickname for the 42nd Regiment of Foot, but was used more and more so that, in 1881, when the 42nd amalgamated with the 73rd Regiment of Foot, the new regiment was named the 'Black Watch (Royal Highlanders)'. The uniform changed over time, but the nickname has been more enduring. The regimental motto was Nemo me impune lacessit (no one attacks me with impunity). The Royal Stewart Tartan was worn by the regimental pipers to reflect the status of 'Royal' regiment.
In 2006 the regiment was merged with the Royal Scots Borderers, the Royal Highland Fusiliers (Princess Margaret's Own Glasgow and Ayrshire Regiment), the Highlanders (Seaforth, Gordons and Camerons) and the Argyll and Sutherland Highlanders to form the Royal Regiment of Scotland, becoming the Black Watch, 3rd Battalion of the Royal Regiment of Scotland.