groundwater n.

Old English grund "bottom; foundation; surface of the earth," also "abyss, Hell," and "bottom of the sea" (a sense preserved in run aground), from Proto-Germanic *grundu-, which seems to have meant "deep place" (source also of Old Frisian, Old Saxon, Danish, Swedish grund, Dutch grond, Old High German grunt, German Grund "ground, soil, bottom;" Old Norse grunn "a shallow place," grund "field, plain," grunnr "bottom"). No known cognates outside Germanic.

Sense of "reason, motive" first attested c. 1200. Meaning "source, origin, cause" is from c. 1400. Electrical sense "connection with the earth" is from 1870 (in telegraphy). Meaning "place where one takes position" is from 1610s; hence stand (one's) ground (1707). To run to ground in fox-hunting is from 1779. Ground rule (1890) originally was a rule designed for a specific playing field (ground or grounds in this sense attested by 1718); by 1953 it had come to mean "a basic rule."

Old English wæter, from Proto-Germanic *watr- (source also of Old Saxon watar, Old Frisian wetir, Dutch water, Old High German wazzar, German Wasser, Old Norse vatn, Gothic wato "water"), from PIE *wod-or, suffixed form of root *wed- (1) "water; wet."

To keep (one's) head above water in the figurative sense is recorded from 1742. Water cooler is recorded from 1846; water polo from 1884; water torture from 1928. Linguists believe PIE had two root words for water: *ap- and *wed-. The first (preserved in Sanskrit apah as well as Punjab and julep) was "animate," referring to water as a living force; the latter referred to it as an inanimate substance. The same probably was true of fire (n.).