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).
Unfortunately, I learned idioms from my grandmother.
That nice woman, a superb cook, used to pronounce parmesan cheese “parmesian cheese”. And she always used the expression, “9th degree”.