Look up diehard in Wiktionary, the free dictionary.
Die hard is a phrase coined by Lieutenant-Colonel William Inglis of the 57th (West Middlesex) Regiment of Foot during the Battle of Albuera. Its original literal meaning has evolved to describe any person who will not be swayed from a belief. The metaphorical sense was first applied almost exclusively to right wing politics, but since the latter half of the 20th century, it has come into general use with wide application.