Body water is the water that makes up more than half of the average human body.
Before the advent of modern physiology, humorism associated Phlegm with water, and my (relatively limited) understanding of the theory suggests that this was supposed to be "balanced" with the other three. I haven't been able to find what that "balance" is, but at least some significant fraction of the human body was water. Humors having been described in the writings of Galen between 130 and 201 AD somewhere is the earliest reference I could find for the human body containing water.
Looking for an earlier scientific reference, I've skimmed De Rerum Natura for references to water, but didn't see it mentioned in particular reference to human anatomy. It describes water as being one of the four traditional elements, but doesn't say whether there's any in the human body.
Is there an earlier reference for the human body's water content? When did western civilization decide humans were wet?