To repair; to solve
A difficult situation; a predicament
“I’m going to fix this” sounds helpful. Reassuring. It implies competence, agency, the confident diagnosis of a problem and its imminent resolution.
Until you realize that the person saying it is often the one who created the fix you are now in.
The contranym turns the promise of solution into the description of the problem with elegant economy. The same word names the crisis and claims to solve it. It is linguistically convenient for anyone whose standard operating procedure involves causing disasters and then offering to rescue people from them.
To be in a fix is to be trapped. To fix something is to free it. The word offers to be both the trap and the way out. This is not a contradiction. This is, historically, a very effective business model.
Note also: “fixed.” Repaired and functional. Also: surgically altered to prevent reproduction. The word does not bother to hide what it thinks the relationship between those two states is.