Our listener Susan Lessian is a Boston transplant who says she still struggles with some "midwesternisms," despite having moved here years ago.
She says, "The one that disturbs me the most is the use of 'can't hardly' for 'can hardly.' Isn't this actually a double negative?"
Susan is right that many usage guides have classified "can't hardly" as a double negative. But the situation is more complicated than that.
First, there's nothing wrong with double-negatives, other than the historical perception that there's something wrong with them, which became a rule. Many languages around the world use double negatives, as do many varieties of English.
So, even if "can't hardly" was a double negative, that wouldn't make it wrong.
"Hardly" is what's called a negative adverbial. It has a negative meaning and approaches negation. For example, "I hardly know her," that's different from "I don't know her." It's the difference between knowing her a little, perhaps just by name, and not knowing her at all.
Sometimes, “hardly” can serve as a full negative, as in this example from the Oxford English Dictionary: "Who do you suppose pays for the $50 billion difference - the Tooth Fairy? Hardly." In other words, that's a hard no. "Hardly" also has a strong negative sense in something like "We can hardly say this week was a warm one." Many people would hear "we can hardly say" as equivalent to "we can't say."
What's interesting is that negatives are weakened when combined with "hardly." Consider the difference between "I can't see the sign" and "I can't hardly see the sign." In the first sentence, the speaker can't see the sign at all, but in the second sentence they can just barely see it.
In its earliest days in Old English, "hardly" meant "vigorously" or "with energy." People hurled stones hardly. Animals bit hardly. It came to involve hardship as in "hardly earned gains," and from there, it comes to mean "not easily or readily." That brings us to the meaning that we're familiar with today of "to an insignificant degree" or "not at all."
As we said, there's nothing wrong with "can't hardly," but it's probably best to avoid this one in formal writing.