'assertory' originates from Latin, specifically the Medieval Latin word 'assertorius', where the root 'asser-' comes from Latin 'asserere' (related to 'asserere' / 'adserere') meaning 'to affirm, to assert'.

'assertorius' (Medieval Latin) passed into Middle English as 'assertorie' or 'assertory' and eventually became the modern English adjective 'assertory'.

Initially it carried the sense 'of or relating to an assertion' in Latin and medieval usage; over time it has come to mean more broadly 'stating or affirming positively' in modern English.