Though some people may think that they are superior to others, and therefore believe only people in Pittsburgh say this, j'eat yet is a sped-up pronunciation of the phrase "did you eat yet?", said throughout the country. It is especially notable in rural South Dakota, Appalachia, and (apparently) Pittsburgh.
When heading north to Brookings, SD on Interstate-29, Bobby saw a sign advertising the Pavillon Bar and Grill. The sign said "J'eat Yet?" and advised Bobby to take exit 130 and turn left. Bobby though to himself, "No, actually, I haven't had a thing to eat since early this morning," and proceded to the Pavilion and ordered a delicious stake and fries.