Just because you haven't heard it before doesn't mean it wasn't used. Musk admittedly overuses the term. Archives show it has been on acronym finder for 27 years 4 months:
https://www.acronymfinder.com/RUD.html The term "RUD" has been used for decades. The belief is it first gained more widespread use after the Saturn 503 S-IVB explosion at the Douglas Sacramento, Calif., Test Facility. This term was attributed to the subsequent MSFC-appointed investigating board's Disassembly Activities (chaired by Kurt Debus, KSC) in 1967.
Also a similar term exists in a 1972 British text:
http://library.sciencemadness.org/library/books/ignition.pdf when you have a turbine spinning at some 4000 rpm, and the clearance between the blades is a few thousandths of an inch, and this sticky, viscous liquid deposits on the blades, the engine is likely to undergo what the British, with precision, call 'catastrophic self-disassembly.' Multiple versions of RUD are used by NASA:
https://history.nasa.gov/monograph45.pdf I heard of the term being used when the Challenger exploded in 1986, but can not find reference of it being used at that time, the earliest reference of RUD for the Challenger event is 1994.
Another more common term is "'
spontaneous rapid disassembly event", used in many analysis of spacecraft engineering and Explosive Ordnance Disposal manuals, training and applied usage. One of the training exercises we worked in the guys said "Sir-dee" multiple times, referring to SRDE.
It was also a common term for F-14 squadrons in the 1970's. RUD was in the manual describing the Pratt & Whitney TF30-P-414A engine shedding.
My neighbor worked for McDonnell Douglas in the 70's and consults for Atlas Pacific now. He stated the term was RUDE:
Rapid, Unplanned Disassembly Event and it has been in the acronym manual at both employers. He literally said to me when Space-X was testing the self-landers, that there are more "Rude-Rockets" in a year now than in a decade when he started as an engineer.
RUD is listed in the 4,339 acronyms and abbreviations related to the Engineering terminology under Academics and Sciences acronym finder.
I'm sure all of them are wrong though, us internet jockeys always know more than the people who actually do the job.