'appertinent' originates from Latin, specifically the verb 'appertinere', where the prefix 'ad-' (seen in the form 'ap-') meant 'to, toward' and 'pertinere' meant 'to pertain, to reach or concern'.

'appertinere' passed into Medieval/Church Latin and then into Anglo-French/Middle French forms (such as 'appertiner'/'appertinent'), from which English borrowed 'appertinent' in the sense 'relating to or belonging to'.

Initially it meant 'to belong to or be related to (something)', and over time it evolved into the adjective meaning 'relevant or pertaining to', a sense that has largely remained.