'predicatively' originates from Latin, specifically the Medieval Latin adjective 'predicativus' (from 'praedicare'), where 'prae-' meant 'before' and 'dicare' meant 'to proclaim/speak'.

'praedicare' in Latin developed into Medieval Latin 'predicativus' meaning 'relating to a predicate', then into Middle English/French forms (e.g. Anglo-Norman/Middle English 'predicatif') and eventually into modern English 'predicative' with the adverbial suffix '-ly' forming 'predicatively'.

Initially connected to the idea of 'proclaiming' or 'declaring' ('to speak before'), it shifted to a grammatical sense meaning 'relating to the predicate of a sentence'; this grammatical sense is the primary modern meaning.