BEAM (from the O. Eng. béam, cf. Ger. Baum, a tree, to which sense may be referred the use of "beam" as meaning the rood or crucifix, and the survival in certain names of trees, as hornbeam), a solid piece of timber, as a beam of a house, of a plough, a loom, or a balance. In the last case, from meaning simply the cross-bar of the balance, "beam" has come to be used of the whole, as in the expression "the king's beam," or "common beam," which refers to the old English standard balance for wholesale goods, for several hundred years in the custody of the Grocers' Company, London. As a nautical term, "beam" was transferred from the main cross-timbers to the side of the ship; thus "on the weather-beam" means "to windward," and a ship is said to be "wide in the beam" when she is wide horizontally. The phrase "to be on one's beam-ends," denoting a position of extreme peril or helplessness, is borrowed from the position of a ship which has heeled over so far as to stand on the ends of her horizontal beams. The meaning of "beam" for shafts or rays of light comes apparently from the use of the word to translate the Latin columna lucis, a pillar of light.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)