In the old days the lookout on a whaling ship would look for signs of a pod of whales. When a whale surfaces after a long period of being under water the first thing it does is to release the air in its lungs with an explosive exhalation through its blowhole, the single nostril on the top of its head. This would be visible as a cloud of water vapor. The lookout would announce this to the sailors and officers on the deck below by shouting "Thar' she blows!" "Thar'" is a dialect version of the standard English "there".