“Wow, is that 9th degree stuff higher than TS SCI?”
“9th degree” is a figure of speech, like “cat got your tongue?” is a figure of speech rather than literal.
Kinda like, “I’m pulling your leg?”
Pretty sure the idiom is “the third degree”
As in the “gave someone the third degree.”
(Which is, itself, a reference to the question-an-answer portion of the 3rd and final degree of masonry.)
An alternative would be to the “nth degree, which is the degree of a polynomial that is the highest exponent for a term with non-zero coefficient in a polynomial expressed in canonical form (ie - as a sum or difference of terms).