The phrase die hard was first used during the Battle of Albuera (1811) in the Peninsular War. During the battle, Lieutenant-Colonel William Inglis of the 57th (West Middlesex) Regiment of Foot was wounded by canister shot. Despite his injuries, Inglis refused to retire from the battle but remained with the regimental colours, encouraging his men with the words "Die hard 57th, die hard!" as they came under intense pressure from a French attack. The 'Die Hards' subsequently became the West Middlesex’s regimental nickname.
The term was later used to deride several senior officers of the Army who sought to maintain the system bequeathed to them by the Duke of Wellington and who strenuously resisted military reforms enacted by Parliament starting in the late 1860s.
In British politics the adjective "die-hard" (best written with a hyphen) was later used to describe those members of the House of Lords who, during the crisis caused by the Lords' rejection of David Lloyd George's "People's Budget" of 1909 refused to accept the diminution of the upper house's powers by the Parliament Act 1911.
It was later used to describe those members of the Conservative Party, including Winston Churchill, who refused to accept any moves towards Indian independence in the 1930s. Again this opposition was powerfully concentrated in the House of Lords. Many of the die hards, though not Churchill, flirted with Oswald Mosley's British Union of Fascists and some even became active sympathisers with Adolf Hitler and called for a negotiated peace in the crisis of 1940.
The term is now commonly used to describe any person who will not be swayed from a belief, and was used as the title of the popular action movie series Die Hard, all starring Bruce Willis as police officer John McClane.