terminal, adj. — Green’s Dictionary of Slang

terminal adj.

[SE terminal, used of disease that will kill the sufferer]

extreme, total.

1972		D. Jenkins Semi-Tough 10: I’m what a lot of your spooks might think of as a red neck with a terminal case of the dumb-ass.
1976		P. Conroy Great Santini (1977) 321: You’ve always been a little screwed up but now it’s beginning to look like you have a terminal case.