The phrase "let alone"
Solution 1:
It is roughly synonymous with the terms "not to mention" or "to say nothing of", except that those terms require a change in tense for the following action.
The sentence "I wouldn't go near a stingray, let alone pet one" implies that the speaker would stop far short of even being in a situation to do what follows the "let alone" clause. They wouldn't go near a stingray, therefore it is out of the question that they would pet one.