A post on Wordorigins reminded me of the curious history of this word, which began as a perfectly ordinary phrase meaning ‘daring to do.’ The OED’s first citation, from Chaucer’s Troilus and Criseyde (c.1374), exemplifies the usage:
v. 837 Troylus was neuere vn-to no wight.. in no degre secounde, In dorryng don [v. rr. duryng do, dorynge to do] þat longeth to a knyght.. His herte ay wiþ þe firste and wiþ þe beste Stod paregal, to dorre don [v. rr. durre to do, dore don] that hym leste.
The online edition (by Skeat) I linked to [Book] v above gives the passage thus:
And certainly in storie it is y-founde,
That Troilus was never un-to no wight,
As in his tyme, in no degree secounde
In durring don that longeth to a knight. [longeth 'is appropriate to']
Al mighte a geaunt passen him of might,
His herte ay with the firste and with the beste
Stood paregal, to durre don that him leste.
[paregal 'fully equal'; durre don that him leste 'dare (to) do what he wanted' (leste = list)]
In the next century, Lydgate in his Chronicle of Troy imitated Chaucer in the following passage:
1430 Lydg. Chron. Troy II. xvi. And parygal, of manhode and of dede, he [Troylus] was to any þat I can of rede, In dorryng [v. rr. doryng(e] do, this noble worþy wyght, Ffor to fulfille þat longeþ to a knyȝt, The secounde Ector.. he called was. [edd. 1513, 1555 In derrynge do, this noble worthy wyght.]
The misprint in the 1513 and 1555 editions seems to have been the crucial factor, obscuring the connection with the verb and enabling Spenser to mistake it for some sort of nominal construction, which he picked up for use in The shepheardes calender (1579):
Oct. 65 For ever who in derring doe were dreade, The loftie verse of hem was loved aye. [Gloss., In derring doe, in manhood and chevalrie.]
That later magpie Sir Walter Scott saw the usage in Spenser, liked it, and stuck it into Ivanhoe (1820):
xxix, Singular.. if there be two who can do a deed of such derring-do. [Note. Derring-do, desperate courage.]
And everybody read Scott, so “derring-do” entered the general vocabulary, to vaguely puzzle readers for centuries to come.