Derring-do is a quirky holdover from Middle English that came to occupy its present place in the language by a series of mistakes and misunderstandings. In Middle English, dorring don meant simply "daring to do." The phrase was misprinted as derrynge do in a 15th-century work by poet John Lydgate, and Edmund Spenser took it up from there. (A glossary to Spenser's work defined it as "manhood and chevalrie.") Literary author Sir Walter Scott and others brought the noun into modern use.
Word History
Etymology
Middle English dorring don daring to do, from dorring (gerund of dorren to dare) + don to do