"to intersect so as to form a figure like the letter X, to cross," 1650s, from Latin decussatus, past participle of decussare "to divide crosswise, to cross in the form of an 'X,'" from decussis "the figure 'ten'" (in Roman numerals, represented by X), also "a large copper coin ten times the value of an as," from decem "ten" (from PIE root *dekm- "ten"). Related: Decussated; decussating; decussation. As an adjective, by 1806.
in physics, 1896, in Roentgen rays "X-rays," in recognition of German physicist Wilhem Conrad Röntgen (1845-1923), who discovered X-rays in 1895. As a unit of exposure to radiation, it is attested from 1922, proposed in French in 1921.