“Ati” is a Chichewa word that literally means “which”. Technically the prefix has to change according to the noun class in question. I am absolutely horrible at noun classes used by Bantu languages, which are akin to the gender of nouns in Romance languages except that there are way more, and they somehow seem to make even less sense.
But it’s also something I hear people say out in Mwambo Traditional Authority all the time in contexts where it makes no sense to be asking “which” about anything. Specifically, about half the time if I say something they repeat it back to me, prefaced by “ati”. If I tell kids “osakoma galimoto” (don’t hit the car*) then at least one almost invariably says “Ati osakoma galimoto!” (a direct translation would be
“which don’t hit the car!”) in an amused and fairly incredulous tone of voice. I get it from adults as well.
Stephen Paas’s excellent Chichewa-English dictionary is of surprisingly little help here. It lists the definitions of “ati” as “ati? 1.which?; anyamata ati? = which boys?; 2.isn’t that right?;”. “Isn’t that right don’t hit the car!” doesn’t make much sense either. So I started running this enigma by my native-Chewa-speaker friends. Their explanation, as I understand it, is that the people around here speak the Chinyanja dialect rather than Chichewa, and this is a local slang term meaning roughly “he/she says”.
So my sense of what’s going on here is that if I try to tell folks that, for example, I’m busy working in their native tongue, they are mostly blown away that this crazy white guy knows how to say stuff that they understand. “He says he’s working!” they yell to their friends, and the subtext is that there’s an mzungu that can actually talk, and that that’s the funniest thing that’s happened all week.