'pertain' originates from Latin, specifically the word 'pertinēre', where 'per-' meant 'completely' or 'through' and 'tenēre' meant 'to hold'.

'pertain' changed from Latin 'pertinēre' into Old French/Middle English forms (e.g. Old French 'pertener', Middle English 'pertainen') and eventually became the modern English word 'pertain'.

Initially, it meant 'to stretch toward or reach; to hold to/extend to', but over time it evolved into its current primary meaning of 'to relate to or be relevant to'.