Nonsense word, primarily used as an adjective but meaning whatever the speaker wants it to mean. Invented by legendary KMPC DJ Gary Owens in the 60s. He used this, along with several others like "creeble" and "grelb," during his radio show, but none were as popular as "insegrevious", which was briefly included in the Funk & Wagnalls Dictionary (until they found out they'd been duped). This in turn gave rise to the phrase "look that up in your Funk & Wagnalls", heard often on Laugh-In, on which Gary was the hand-on-the-ear announcer in the booth.
The other story about Chuck Ferguson and "Simon Says" is clearly wrong, as Laugh-In started in 1968 and "insegrevious" and the Funk & Wagnalls listing had already happened then. The "Simon Says" pilot didn't happen until 1971.
"Look at that klarvy murrish creeching in the nern. Fnork! How insegrevious can you get?"