Ain’t am not bad English.
In fact, ain’t is quite good English. It’s just not generally accepted English.
Let’s look at how the word is constructed. “I ain’t” is a contraction of “I am not.” It avoids the clumsy contraction “amn’t” (which makes a pretty good onomatopoeic representation of a lumpy gulp). And it follows the construction of words like “won’t” and “shan’t,” in which ending consonants are swallowed (maybe in that big lumpy gulp). Ain’t exhibits a change of vowel pronunciation just like won’t, which actually features a change of the vowel itself.
I submit that that very vowel change may have led in part to the disdain for ain’t (should we call that “disdain’t”?). In “I ain’t,” the pair of shrill long vowels in combination— aye ay —grates the ear, and sends the speaker’s tongue into a bit of gymnastics. Would the word be more acceptable if the A were to retain its short-A pronunciation, as in “I an’t”? Perhaps, although we suddenly have another ant/aunt homonym. As well, saying “I an’t” perpetuates the voicing of double vowels, whereas the alternate contraction, “I’m not,” absorbs one of the vowels and allows easy flow from vowel to consonant. “I an’t” is simply more difficult to mouth, a problem that, say, converting “I can not” to “I can’t” doesn’t face.
Interestingly, my “suggestions” above trace the actual evolution of I am to I amn’t to I an’t to I ain’t. How did we get to the
sharp-syllabled ain’t? Sounds a bit strange, perhaps, until you consider another colloquial pronunciation: I cain't.
Again, it’s a matter of word fashions, the reason reckon sounds backwoods to Americans even though it’s very much frontwoods to the Brits.
If you use this perfectly respectable English word ain’t, use it boldly. It ain’t for the fain’t of heart.