'nonassertoric' is formed in English from the negative prefix 'non-' (from Latin 'non', meaning 'not') attached to 'assertoric' (relating to assertion).

'assertoric' ultimately derives from Latin roots related to 'asserere'/'asser-', meaning 'to affirm' (via Medieval/Scholarly Latin forms such as 'assertorius' or similar), and entered English as 'assertoric'; the modern compound 'nonassertoric' was created by adding the productive English prefix 'non-'.

Originally related to 'assert' and meanings of 'affirming' or 'asserting'; with the addition of 'non-' the meaning became specifically 'not asserting' and was adopted in technical fields (logic, semantics) to label non-assertive utterances.