The only time we use the word "tummy" around my house is when we're looking for a delicate way to describe gastrointestinal distress. "Tummy troubles" describes things like diarrhea and gas, but by using that phrase nobody has to outright say "diarrhea and/or gas." It started when our girls were little and just stuck. But aside from that, nobody talks about "tummies" here...because our girls are not children anymore, and neither are we.