I just used this phrase in a Good Reads comment on Anthony Horowitz's Point Blank a little bit ago. Though I double-checked before posting, I did know it was 'derring do', not 'daring do', as I'm sure it has been spelled many a time. But I have no idea of where the phrase comes from. Nor, when I get right down to it, do I know what it really means. I know what I mean by it, which is roughly 'doing daring things'. But as I think about it,that's probably too easy. 'Derring' and 'daring' are probably not just variants of the same word. The only thing I can think of us is that derring might be from German or possibly the Nordic. The latter is probably only because I'm thinking of the word 'herring'. Well, before I drift too hopelessly out to sea, let's find out...
For once, I seem to have been pretty much right the first time round, which at first was a bit of a letdown. I guess it's actually more properly 'derring-do', but that's minor stuff--it still means something along the lines of 'heroic daring'. However, digging a little further often leads to more rewarding things, and it's certainly the case with this word or phrase. It is in fact, a word borrowed, passed along and variously shaped and misinterpreted by some of the brightest lights of English literature. It first finds it way into print in Chaucer's Troylus and Criseyde as 'durring don'--daring to do; becomes 'dorryng do' through the poet John Lydgate in a nod back to Chaucer; is misprinted later as 'derrynge do' and then misinterpreted by Edmund Spenser, who much like me (though in this one sense only) thought he was encountering a different word and took it to mean 'brave actions'-- though he too changed it a little to 'derring doe'--which Walter Scott then grabbed up for Ivanhoe, from which his version 'derring-do' then became part of the common parlance.
I pretty much lifted all that from phrases.org.uk, by the way, so have a look if you would like a fuller and very lively account of the above.
I was also pleased to learn that 'derring-do' is what the OED calls a 'psuedo archaism'. We like to think of those times when men were men and acted out brave feats of derring-do, except that, well, they didn't exactly. Or at least they didn't know that was what they were doing.
Chaucer, Lydgate, Spenser, Scott--great men and great writers--but not the most meticulous of spellers, I'm thinking.