fountainhead (n.)

early 15c., "spring of water that collects in a pool," from Old French fontaine "natural spring" (12c.), from Medieval Latin fontana "fountain, a spring" (source of Spanish and Italian fontana), from post-classical noun use of fem. of Latin fontanus "of a spring," from fons (genitive fontis) "spring (of water)," from PIE root *dhen- (1) "to run, flow" (source also of Sanskrit dhanayati, Old Persian danuvatiy "flows, runs").

The extended sense of "artificial jet of water" (and the structures that make them) is first recorded c. 1500. Hence also fountain-pen (by 1823), so called for the reservoir that supplies a continuous flow of ink. "A French fountain-pen is described in 1658 and Miss Burney used one in 1789" [Weekley]. Fountain of youth, and the story of Ponce de Leon's quest for it, seem to have been introduced in American English by Hawthorne's "Dr. Heidegger's Experiment" (January 1837).

"Did you never hear of the 'Fountain of Youth'?" asked Dr. Heidegger, "which Ponce de Leon, the Spanish adventurer, went in search of two or three centuries ago?"

Old English heafod "top of the body," also "upper end of a slope," also "chief person, leader, ruler; capital city," from Proto-Germanic *haubid (source also of Old Saxon hobid, Old Norse hofuð, Old Frisian haved, Middle Dutch hovet, Dutch hoofd, Old High German houbit, German Haupt, Gothic haubiþ "head"), from PIE root *kaput- "head."

Modern spelling is early 15c., representing what was then a long vowel (as in heat) and remained after pronunciation shifted. Of rounded tops of plants from late 14c. Meaning "origin of a river" is mid-14c. Meaning "obverse of a coin" (the side with the portrait) is from 1680s; meaning "foam on a mug of beer" is first attested 1540s; meaning "toilet" is from 1748, based on location of crew toilet in the bow (or head) of a ship.

Synechdochic use for "person" (as in head count) is first attested late 13c.; of cattle, etc., in this sense from 1510s. As a height measure of persons, from c. 1300. Meaning "drug addict" (usually in a compound with the preferred drug as the first element) is from 1911.

To be over (one's) head "beyond one's comprehension" is by 1620s. To give head "perform fellatio" is from 1950s. Phrase heads will roll "people will be punished" (1930) translates Adolf Hitler. Head case "eccentric or insane person" is from 1966. Head game "mental manipulation" attested by 1972.