bridgehead n.

[structure that affords passage over a ravine or river] Middle English brigge, from Old English brycge, from Proto-Germanic *brugjo (source also of Old Saxon bruggia, Old Norse bryggja, Old Frisian brigge, Dutch brug, Old High German brucca, German Brücke), from PIE root *bhru "log, beam," hence "wooden causeway" (source also of Gaulish briva "bridge," Old Church Slavonic bruvuno "beam," Serbian brv "footbridge").

The original notion is of a beam or log laid down for crossing. Compare Old Church Slavonic mostu, Serbo-Croatian most "bridge," probably originally "beam" and a loanword from Germanic, related to English mast (n.1). For the vowel evolution, see bury.

The meaning "bony upper part of the nose" is from early 15c.; of stringed instruments from late 14c. The bridge of a ship (by 1843) originally was a "narrow raised platform athwart the ship whence the Captain issues his orders" [Sir Geoffrey Callender, "Sea Passages"].

Bridge in steam-vessels is the connection between the paddle-boxes, from which the officer in charge directs the motion of the vessel. [Smyth, "The Sailor's Word-book," 1867]

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.

also beachhead, "a position on a beach taken from the enemy by assault from the sea and used as a base for further attack," 1940, in reference to German military tactics in World War II, from beach (n.) + head (n.). On the model of bridgehead, but the image doesn't quite work; worse is the attempt at airhead.