A definição real pode ter sido uma piada, taquigrafia de teclado ou referência obscura. A Wikipédia tem isto:
The name dd may be an allusion to the DD statement found in IBM's Job Control Language (JCL),[3] where the initials stand for "Data Description."[4] The command's syntax resembles the JCL statement more than it does other Unix commands, so the syntax may have been a joke.[3] Another explanation for the command's name is that "cc" (for "convert and copy", as in the command's description) was already taken by the C compiler. It is also jokingly said that dd stands for "disk destroyer" or "delete data", since when used for low-level operations on hard disks, a small mistake such as reversing the input file and output file parameters could result in the loss of some or all data on a disk.
Fonte: dd (unix) em en.wikipedia.org
Esta página no codecoffee tem algumas discussões adicionais.
Muitos sysadmins criam seus próprios mnemônicos para comandos como dd (1), e, neste caso, parece que praticamente tudo vale!