It’s defined as a rhyming euphemism from the Appalachians on this linguistics page I bookmarked for something else, as it happens (scroll down to section 4), but gives no explanatory info…

http://pandora.cii.wwu.edu/vajda/ling201/test3materials/AmericanDialects.htm


Again, this site doesn’t provide anything very definitive, but is quite interesting!

http://www.uncc.edu/charleston/our writings/stories_text.htm#Lowcountry Colloquialisms and Patois


Then of course there’s our old favourite topic from way back:

Slow down Pa!
Sakes alive!
Ma missed signs
four and five.
Burma Shave.
http://www.the-daily-record.com/past_issues/02_feb/000227dr1.html

It also seems to have been a much-favoured phrase by Sarah Orne Jewett – is her writing as twee as some extracts make her sound?