Stiffness, Stiffening
Noun and する verb
こうちょく
心理的硬直は問題を解決する妨げになると思われる。
I believe that psychological rigidity can hinder problem solving.
社会の硬直化は解決しなければならない問題です。
The rigidity of society is a problem that we need to fix.
死後どのぐらいで死後硬直が起きるんでしょうか。
I wonder how long after death rigor mortis sets in.
A stiff fix is when you fix something to be stiff, which leads to rigidity, stiffness, or stiffening. This word feels pretty formal when used as a noun. You can also add している or した to mean "rigid," but it doesn't mean that on its own.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.