“Ati” is a swahili word. It is an interjection. It’s an expletive expression of surprise and is also used to call attention.
In Kenya it is used to correspond to english expressions like “I beg your pardon”. Source: Mordern Swahili Grammar
This photo was taken in December of 2013. I went dressed as a Christmas tree :). I bought this t-shirt from Chilli Mango.