[UNIX: from IBM JCL]
This was originally the name of a UNIX copy command with special options suitable for block-oriented devices. Often used in heavy-handed system maintenance, as in "Let's dd the root partition onto a tape, then use the boot PROM to load it back on to a new disk".
The UNIX 'dd(1)' was designed with a weird, distinctly non-UNIXy keyword option syntax reminiscent of IBM System/360 JCL (which had a similar DD command); though the command filled a need, the interface design was clearly a prank. The jargon usage is now very rare outside UNIX sites and now nearly obsolete even there, as 'dd(1)' has been deprecated for a long time (though it has no exact replacement).
Replaced by BLT or simple English 'copy'.