'spec' originates from English as an informal abbreviation of 'specification', where 'specification' ultimately derives from Latin roots related to 'specere'/'species' meaning 'to look' or 'appearance'.

'specification' comes from Late Latin 'specificatio'/'specificare' (from Latin 'species' and related roots); in modern English technical and commercial usage the clipped form 'spec.' appeared and in 20th-century colloquial and industry usage this shortened form became the standalone word 'spec'.

Initially it referred to the full word 'specification' (a detailed description); over time the clipped form 'spec' broadened colloquially to refer not only to specification documents but also (by analogy or separate clipping) to 'specs' meaning 'spectacles' and to usages related to 'speculation' (as in 'on spec').