OK I guess I made you wait long enough. This was I think explained in my first ever podcast on Podnutz, but I don't think I've ever explained it in text.
First, "d" is in front of other apps for two reasons. First because the base line code for most of my apps generic functions was originally written for d7, so being a spawn of d7 I wanted to keep something in the name. I also am horrible at naming things, so I decided that shoving that "d" in front of an otherwise generic sounding app name would make it unique enough to call it "mine" yet still get the message plainly across as to what the app actually does (e.g. dMaintenance)
d7 probably originally does refer to the chord, I'm not sure. d7 as I knew it first was a song from "Is This Real?" by The Wipers, an early punk band headed by Greg Sage that was highly influential with a lot of later grunge bands. d7 was covered most notably by Nirvana, who also covered their song Return of the Rat, and these were on Fourteen Songs for Greg Sage and The Wipers, a variety compilation with Hole and a bunch of lesser known names. Yes believe it or not, I had heard of The Wipers before Nirvana...
Completely by chance I was listening to The Wipers when I was trying to think of the name for d7, though I don't think I was listening to that song or probably even that album, but d7 struck me - and I was looking for clues in anywhere I could find them. Since as I said, I'm horribly unoriginal with naming, it was far better than something dumb sounding and generic like "Virus Killer" or even what I had initially called my real first app, which later became d7's Offline Operations tab; that was called "AutoTBC" and that horrible generi-acronym stands for "Automated Tech Bench Computer" ...
My actual logic behind using "D7" was this: A name that makes no sense, with a number in it for absolutely no damn reason, is very "hip" and "cool" with the young'ins. Even for the older folk it's just mysterious and makes you want to know WTF it is about. At the time d7 was conceived, the hottest thing on the music scene is for every new band to have a number in their name, for absolutely no damn reason other than it sold records.
Though I never understood why the name+number thing seemed to sell so well, I did recognize that d7 was bound to become popular if tech paralleled music in any way, because more no-talent ass-clowns than ever before were rising to the top of the radio playlists fitting that name+number description. (Not at all saying d7 is a no-talent ass-clown, but to be honest I didn't have much faith in my ideas and product at the time just because I've always undervalued my work, so I figured I needed all the help I could get by using a catchy and mysterious name.)
BTW though I could never find out for sure one way or the other, I really think it certainly had a positive effect on d7's popularity.