blockhead n.

"solid piece," early 14c., blok, blokke, "large solid piece of wood," usually with one or more plane faces, from Old French bloc "log, block" of wood (13c.), which is from a Germanic source such as Middle Dutch bloc "trunk of a tree," Old High German bloh (from PIE *bhlugo-, from *bhelg- "a thick plank, beam;" see balk (n.)).

The word was generalized by late 15c. to any solid piece. The meaning "solid mass of wood, the upper surface of which is used for some purpose" is from late 15c., originally the executioner's block where the condemned were beheaded. The meaning "stump where a slave stood to be sold at auction" is from 1842. The sense of "mold on which something is shaped, or placed to keep its shape," typically a hat or wig, is from 1570s; the meaning "head" (generally disparaging) is from 1630s, perhaps an extension of this. To knock (someone's) block off "thrash, beat" is by 1923.

The meaning "grooved pulley in a wooden case" (used to transmit power and change the direction of motion by means of a rope) is from c. 1400. Hence block and tackle (1825; see tackle (n.)). The meaning in city block is 1796, from the notion of a "compact mass" of buildings.

BLOCK. A term applied in America to a square mass of houses included between four streets. It is a very useful one. [Bartlett]

Later of a portion of a city enclosed by streets, whether built up or not.

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.