The word 'vigor' entered English in the 14th century from Anglo-French and Old French 'vigor', meaning 'force, strength'. This, in turn, derived directly from Latin 'vigor', signifying 'liveliness, activity, force'. The Latin noun stems from the verb 'vigēre', meaning 'to be lively, to thrive', which is built on the Proto-Indo-European root weg- meaning 'to be strong, lively'. The semantic journey is clear and linear: from the ancient concept of being lively and thriving, to the classical Latin sense of active force, and finally to the modern English abstract meaning denoting robust physical or mental energy and intensity.

The word 'vigor' is a simplex noun borrowed whole from Latin. It is not productively segmented into smaller, meaningful morphemes in modern English spelling.