In simplest terms back atcha' means, "Same to you." It's a positive connotation.
"Baby, you look so stylin'!" to which your baby says, "back atcha'.'"
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What you smilingly text to your chat-buddy in response to his affectionately sending you assorted "hugz 'n' kissiz" over instant messenger.
You: (blinky eyes, lips) shnoogle-wooglez (heart, flower)
Me: right back atcha :)