(The). The 5th Foot. This sobriquet was given to the regiment during the Peninsular War. The “Old and Bold Fifth,” the Duke of Wellington's Body-guard, is now called the “Northumberland Fusiliers.” What a terrible vexation must the abolition of the time-honoured names of our old regiments have been to our army!
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894