bulkhead (n.)

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 baulk, Middle English balke, from Old English balca "ridge, bank," from or influenced by Old Norse balkr "ridge of land," especially between two plowed furrows, both from Proto-Germanic *balkon- (source also of Old Saxon balko, Danish bjelke, Old Frisian balka, Old High German balcho, German Balken "beam, rafter"), from PIE root *bhelg- "beam, plank" (source also of Latin fulcire "to prop up, support," fulcrum "bedpost;" Lithuanian balžiena "cross-bar;" and possibly Greek phalanx "trunk, log, line of battle"). Italian balco "a beam" is from Germanic (see balcony).

In old use especially "an unplowed strip in a field, often along and marking a boundary." The modern senses are figurative, representing the balk as a hindrance or obstruction (see balk (v.)), or else the notion of "a piece missed in plowing" as "a blunder, a failure." Hence, in baseball, "a motion made by the pitcher as if to deliver the ball, but without doing so," attested from 1845, probably from the plowing sense.