'avowedly' originates from English, specifically the adjective 'avowed', where 'avowed' is the past participle of the verb 'avow' meaning 'to declare openly'.

'avowed' changed from Middle English (past participle forms from Middle English 'avowen') and from Old French 'avouer', and eventually produced the modern English adjective 'avowed' and the adverb 'avowedly'.

Initially it meant 'declared or acknowledged openly', and over time this basic sense has been retained in the modern adverb 'avowedly' meaning 'in an openly declared way'.