It ain't nothin'

I've talked about 'double negation' before, by which I really mean negative concord: using two negative elements in one clause to give a single negative meaning. I've pointed out how it makes no sense to argue against it on the grounds of it being illogical (it isn't), and I usually say that in context, you can always understand what the speaker means. That's more or less true in speech, when you have intonation to help you out. But in writing, without that clue, sometimes the context isn't quite enough. I found this example in a friend's facebook post:

15 days in jail, which ain't nothin'. 

The context really didn't tell me if 15 days was a lot or not much for the crime in question, and so without the intonational cues (in my crude non-expert terms, whether you'd give stress to both ain't and nothin' or just to nothin') I couldn't tell if he meant that 15 days was a significant time, or that 15 days was nothing at all.