{cat} or {BLT}. Originally the name of 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(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); 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). The term has been
displaced by {BLT} or simple English `copy'.

:DDT: /D-D-T/ /n./ 1. Generic term for a program that assists
in debugging other programs by showing individual machine
instructions in a readable symbolic form and letting the user
change them. In this sense the term DDT is now archaic, having
been widely displaced by `debugger' or names of individual
programs like `adb', `sdb', `dbx', or `gdb'.
2. [ITS] Under MIT's fabled {{ITS}} operating system, DDT (running
under the alias HACTRN, a six-letterism for `Hack Translator') was
also used as the {shell} or top level command language used to
execute other programs. 3. Any one of several specific DDTs (sense
1) supported on early {DEC} hardware. The DEC PDP-10 Reference
Handbook (1969) contained a footnote on the first page of the
documentation for DDT that illuminates the origin of the term:

Historical footnote: DDT was developed at MIT for the PDP-1
computer in 1961. At that time DDT stood for "DEC Debugging