The actual definition may have been a joke, keyboard shorthand, or obscure reference. Wikipedia has this:
The name dd may be an allusion to the DD statement found in IBM's Job
Control Language (JCL), where the initials stand for "Data
Description." The command's syntax resembles the JCL statement more
than it does other Unix commands, so the syntax may have been a
joke. 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.
Source: dd (unix) on en.wikipedia.org
This page on codecoffee has some additional discussion.
Many sysadmins have come up with their own mnemonics for commands like dd(1), and, in this case, it seems pretty much anything goes!