* A made-up phrase: The person using it might be trying to create a unique expression to convey a specific idea.
* A slang term: It might be a regional or niche slang expression with a specific meaning within a particular community.
* A metaphor: The speaker might be using "crack back" metaphorically, implying something cannot be broken, defeated, or overcome.
To understand the meaning, you need more context:
* Who said it? Their background and the situation might provide clues.
* What was the context? What was the speaker talking about before or after using this phrase?
* What was the speaker's tone? Was it serious, humorous, or threatening?
Without more information, it's difficult to determine the exact meaning of "nothing can crack back." If you provide more context, I can try to help you understand its meaning.