The act of putting on a sugary-
sweet and polite persona to someone's face (or out in public), but being snide, catty, and mean when their back is turned.
OR
The
fake friendliness and hospitality demonstrated (particularly in the
American South/ Southeast) to total strangers, exemplified by "Southern hospitality".
"Watch out: my
mother-in-law will be very charming when she offers you some of her sweet
tea, but she's just Paula Deening and will start complaining about
everything from your clothes to your accent just as soon as you're out of earshot."