'spectre' originates from Latin, specifically the word 'spectrum', where 'specere' meant 'to look' or 'to behold'.

'spectre' entered English via Anglo-French/Old French (e.g. 'spectre') and Middle English, evolving from Latin 'spectrum' into the modern English 'spectre'.

Initially, it referred to an appearance or apparition ('an appearance to the eye'); over time it came to mean specifically a ghost or a haunting image and, by extension, a metaphorical threat.