The word 'vapour' entered English in the late 14th century from Anglo-French 'vapour', which itself came from Latin 'vapor' meaning 'steam, exhalation, heat'. The Latin term is of uncertain deeper origin but may be connected to the concept of exhaling or blowing. The core semantic journey is from the physical phenomenon of steam or warm exhalation observed in nature to the modern scientific and general term for a visible mist or a substance in a gaseous state. This evolution reflects a continuity of the core idea of a diffused, airborne substance.

The word 'vapour' is a simplex loanword from Anglo-French and Latin. It does not contain separable, meaningful morphemes within its modern English spelling.