I know very well what the command does, but
info dd tell me: 'Convert and copy a file', as does GNU Coreutils.
Google says its an abbreviation of everything between medicine and bad webchat slang; except someone saying it means 'data destroyer', something used in PC forensics - I'd be horrified if my dd destroyed my data!
Any insight? :-)
Update: Of course I had to check the jargon file:
The Unix dd(1) was designed with a weird, distinctly non-Unixy keyword option syntax reminiscent of IBM System/360 JCL (which had an elaborate DD ‘Dataset Definition’ specification for I/O devices)
Still sounds pretty ambiguous, but then it says:
though the command filled a need, the interface design was clearly a prank.