Thin Red Line
(The). The old 93rd Highlanders were so described at the battle of Balaclava by Dr. W. H. Russell, because they did not take the trouble to form into square. “Balaclava” is one of the honour names on their colours, and their regimental magazine is named The Thin Red Line.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894