Wordorigins.org

Ghost is of Germanic origin. It dates back to the Old English period and the word gast. The general sense of the word is a spirit or soul and it is used in various specific contexts.

The earliest attested context is that of the soul of a dead person. From Bede’s Death-song, c.735:

Huaet his gastae godaes aeththa yflaes aefter deothdaege doemid uueorthae.
(Lo! His ghost, of good or of evil, after death-day becomes judged.)