O.E. gast "soul, spirit, life, breath," from P.Gmc. *ghoizdoz (cf. O.S. gest , O.Fris. jest, M.Du. gheest, Ger. Geist "spirit, ghost"), from PIE base *ghois- "to be excited, frightened" (cf. Skt. hedah "wrath;" Avestan zaesha- "horrible, frightful;" Goth. usgaisjan , O.E. gæstan "to frighten"). This was the usual W.Gmc. word for "supernatural being," and the primary sense seems to have been connected to the idea of "to wound, tear, pull to pieces." The surviving O.E. senses, however, are in Christian writing, where it is used to render L. spiritus , a sense preserved in Holy Ghost . Modern sense of "disembodied spirit of a dead person" is attested from c.1385 and returns the word toward its ancient sense. Most IE words for "soul, spirit" also double with ref. to supernatural spirits. Many have a base sense of "appearance" (e.g. Gk. phantasma ; Fr. spectre ; Pol. widmo , from O.C.S. videti "to see;" O.E. scin , O.H.G. giskin , originally "appearance, apparition," related to O.E. scinan , O.H.G. skinan "to shine"). Other concepts are in Fr. revenant , lit. "returning" (from the other world), O.N. aptr-ganga , lit. "back-comer." Bret. bugelnoz is lit. "night-child." L. manes , lit. "the good ones," is a euphemism. The gh- spelling appeared c.1425 in Caxton, influenced by Flem. and M.Du. gheest, but was rare in Eng. before c.1550. Sense of "slight suggestion" (in ghost image, ghost of a chance, etc.) is first recorded 1613; that in ghost writing is from 1884, but that term is not found until 1927. Ghost town is from 1931. Ghost in the machine was Gilbert Ryle's term (1949) for "the mind viewed as separate from the body."
Online Etymology Dictionary, © 2010 Douglas Harper