Etymology
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bam (interj.)

imitative of the sound of a hard hit, first recorded 1922 (from 1917 as the sound of an artillery shell bursting). Middle English had a verb bammen "to hit or strike" (late 14c.). A literary work from c. 1450 represents the sound of repeated impact with Lus, bus! las, das!, and Middle English had lushe "a stroke, blow" (c. 1400); lushen "to strike, knock, beat" (c. 1300). Bam also was an old slang shortening of bamboozle (18c.).

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beam (n.)

Old English beam, "living tree," but by late 10c. also "rafter, post, ship's timber," from Proto-Germanic *baumaz "tree" (source also of Old Frisian bam "tree, gallows, beam," Middle Dutch boom, Old High German boum, German Baum "tree," and perhaps also (with unexplained sound changes) Old Norse baðmr, Gothic bagms). This is of uncertain etymology (according to Boutkan probably a substrate word). The shift from *-au- to -ea- is regular in Old English.

The meaning "ray of light" developed in Old English, probably because beam was used by Bede to render Latin columna (lucis), the Biblical "pillar of fire." The meaning "directed flow of radiation" is from 1906. To be on the beam "going in the right direction" (1941) originally was an aviator's term for "to follow the course indicated by a radio beam."

The nautical sense of "one of the horizontal transverse timbers holding a ship together" is from early 13c., hence "greatest breadth of a ship," and slang broad in the beam, by 1894 of ships; of persons, "wide-hipped," by 1938.

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