Not/never in a hundred, etc. years
Definition 1 of 1
Pro Tip 1/3Strong Refusal
Used to strongly deny willingness or likelihood of an action or event. 
Start a snake farm? Not in a hundred years!
Pro Tip 2/3Emphasis on Improbability
Stresses how an action or event is extremely unlikely to occur. 
Not in a hundred years would that timid cat attack the dog.
Pro Tip 3/3Colloquial Expression
Commonly used in informal conversation rather than formal writing. 
Hike up the mountain in this weather? Never in a hundred years!