mast-head (n.)

"long pole on a ship, secured as the lower end to the keel, to support the yards, sails, and rigging in general," Old English mæst, from Proto-Germanic *mastaz (source also of Old Norse mastr, Middle Dutch maste, Dutch, Danish mast, German Mast), from PIE *mazdo- "a pole, rod" (source also of Latin malus "mast," Old Irish matan "club," Irish maide "a stick," Old Church Slavonic mostu "bridge").

The single mast of an old ship was the boundary between the quarters of the officers and those of the crew, hence before the mast "serving as an ordinary seaman" in the title of Dana's book, etc.

In all large vessels the masts are composed of several lengths, called lower mast, topmast, and topgallantmast. The royalmast is now made in one piece with the topgallantmast. A mast consisting of a single length is called a pole-mast. In a full-rigged ship with three masts, each of three pieces, the masts are distinguished as the foremast, the mainmast, and the mizzenmast; and the pieces as the foremast (proper), foretopmast, foretopgallantmast, etc. In vessels with two masts, they are called the foremast and mainmast; in vessels with four masts, the aftermast is called the spanker-mast or jigger-mast. [Century Dictionary, 1895]

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.