Etymology
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spar (n.1)

early 14c., "rafter;" late 14c., "stout pole," from or cognate with Middle Low German or Middle Dutch sparre, from Proto-Germanic *sparron (source also of Old English *spere "spear, lance," Old Norse sperra "rafter, beam," German Sparren "spar, rafter"), from PIE root *sper- (1) "spear, pole" (see spear (n.1)). Nautical use, in reference to one used as a mast, yard, boom, etc., dates from 1630s. Also borrowed in Old French as esparre, which might be the direct source of the English word.

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axle-tree (n.)

also axletree, "bar or beam fitted crosswise under the body of a carriage and having wheels fitted to the ends," c. 1300; see axle (n.) + tree (n.).

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cat-head (n.)

"beam projecting from each side of the bows of a ship to hold the anchor away from the body of the ship," 1620s, from cat (n.) in some obscure sense + head (n.).

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tram (n.)

c. 1500, "beam or shaft of a barrow or sledge," also "a barrow or truck body" (1510s), Scottish, originally in reference to the iron trucks used in coal mines, probably from Middle Flemish tram "beam, handle of a barrow, bar, rung," a North Sea Germanic word of unknown origin. The sense of "track for a barrow, tramway" is first recorded 1826; that of "streetcar" is first recorded 1879, short for tram-car "car used on a tramway" (1873).

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caber (n.)

pole used in housebuilding, especially as an object tossed in the Highland games, 1510s, from Gaelic cabar "pole, spar," cognate with Irish cabar "lath," Welsh ceibr "beam, rafter."

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radiation (n.)

mid-15c., radiacion, "act or process of emitting light," from Latin radiationem (nominative radiatio) "a shining, radiation," noun of action from past-participle stem of radiare "to beam, shine, gleam; make beaming," from radius "beam of light; spoke of a wheel" (see radius).

Meaning "rays or beams emitted" is from 1560s. Meaning "divergence from a center" is 1650s. In modern physics, "emission or transmission of energy in the form of waves or particles," especially in reference to ionizing radiation, from early 20c.

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girder (n.)

"main supporting wooden beam that carries flooring," 1610s, agent noun from gird, on notion of something that "holds up" something else. Used of iron bridge supports from 1853.

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bridge (n.1)

[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]
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hornbeam (n.)

type of small tree, 1570s, from horn (n.) + beam (n.) "tree," preserving the original sense of the latter word. The tree so called in reference to its hard wood, which somewhat resembles horn.

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arbor (n.2)

"main support or beam of a machine," 1650s, from Latin arbor, arboris "tree," from Proto-Italic *arthos, which de Vaan derives from PIE *herdhos "height, uprightness," from root *eredh- "to grow, high" (see ortho-).

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