Is NIB "New in Box" or "Not in Box"?

I have been selling items on eBay that are in their original packaging as NIB because I thought it meant "New in Box", but as I was sending something, my Dad asked me why I wrote NIB when it was in a box. He told me he always thought it meant "Not in Box". It hadn't occurred to either of us that both use the same abbreviations... Which is it? Is he right or am I?