Heroically brave exploits. “Derring” comes from “daring, and “do” is related to “done.” Geoffrey Chaucer originated the phrase in his poem Troilus and Criseyde; it was picked up by Edmund Spenser and again by Sir Walter Scott in Ivanhoe. If you come across it in contemporary speed or writing, you're more than likely to hear it in the longer phrase “deeds of derring do.”