A Unix copy command with special options suitable for block-oriented devices; it was 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" 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); 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" has been deprecated for a long time (though it has no exact replacement). The term has been displaced by BLT or simple English "copy".
[Jargon File]
Computing Dictionary