A cross between the standard englih contraction "can't"(can not) and non-standard english contraction "ain't" (am not). This combination of contraction forms a powerful word encompassing "can not", "am not", "is not", "are not", and "have not".
"Cain't" is not the same as "can't." "Can't" as you know is a contraction for "cannot." Adding the "i" means that whatever cannot be done is WANTING to be done but is impossible to do so. It "ain't possible." and this is usually due to the circumstances surrounding it.
This word, generally pronounced cain', without the t, is the negative of "can" for those who habitually drop a stop at the end of a word if a consonant follows, as in southern American English. The vowel has to be shifted from a short a to a long a in order to make the distinction.