See a doctor.
Insatiable thirst even after drinking copious amounts of water generally means there is a chemical imbalance in the blood.
Most commonly, this occurs in people who are diabetic and do not yet know they have the disease. Often when a person is first diagnosed, they will report constant thirst as well as fatigue, malaise
, change in appetite
, and frequent urination
, all due to high blood sugar
Answer assisted by Cole Livingston, Medical Student