"Can hardly wait" versus "can't hardly wait"

The phrase "I can't hardly wait" is incorrect.

I suspect it is the result of a confusion between:

I can't wait

and

I can hardly wait

which are both correct.

I can't hardly wait

doesn't make sense: it would mean "I don't find it hard to wait", which is probably not what is meant.

Both are correct in the sense that a native English speaker will understand what you meant when you use either.

The operative word in both expressions is hardly, which has multiple meanings:

1. with difficulty
2. barely, scarcely

The first sense of the word is what causes some to say can't hardly wait is incorrect: if you can't wait with difficulty, you must mean you can wait a good deal.

But in reality, it's the second sense of hardly that being used. Someone who can hardly wait has just enough willpower to wait out whatever it is. Someone who can't hardly wait doesn't even have that amount.

(Note: Wikitionary even claims the first sense is obsolete)

They're interchangeable -- "can't hardly" is a regional/dialectical variant. The proscription against emphatic double negatives is purely artificial in English; they have been around as long as the English language itself.

Ne con ic noht singan; and ic for žon of þeossum gebeorscipe ut eode ond hider gewat, for þon ic naht singan ne cuðe.

Twice in that sentence (from the prologue to Cædmon's Poem from the Venerable Bede), Cædmon says the equivalent of "I can't sing nothing". Similar examples can be found in Chaucer, Shakespeare, the letters of Abigail Adams, and so on.

Double negatives used for emphasis are as idiomatic to English as split infinitives. Use whichever version you're comfortable with -- unless you're turning a composition in to be marked -- and realise that others do not need to be corrected out of it.

Even if the double negative in "can't hardly wait" is logically, and I'm almost sure gramatically, incorrect, I think many would understand what the speaker means.

In Spanish, my native tongue, using double negatives is quite common and there are expressions such as "no veo a nadie", literally "I don't see nobody," which are clearly understood as "I don't see anybody."