Chaucer, when writing Troilus and Criseyde in 1374, used the language of his day in describing his hero as “in no degre secounde in dorrying don that longeth to a knyght.”
He meant that Troilus was second to no one in “daring to do” that befitting to a knight. But a succeeding poet, John Lydgate in 1430, who borrowed extensively from Chaucer thought that dorrying don was some manly quality, and said that Troilus was a second Hector in dorrying do.
At least he wrote it dorrying do, changing it slightly from Chaucer’s dorrying don. But greater mischief was done when Lydgate’s book was reprinted after his death, because the printer changed it still further to derryinge do.
The poet Edmund Spenser completed the misinterpretation in 1579. Relying upon the language of Lydgate’s reprint, not only did he use derring doe, the reformed spelling of his time, in the text of The Shepheardes Calendar, but he told, in a glossary, what he understood it to mean, “manhood and chevalrie.”
That is the sense in which our poets and romanticists still use it, though arrived at deviously from what Chaucer wrote.