'avengingly' ultimately derives from the verb 'avenge', which comes from Old French 'avengier' (also 'avenger') and is related to Latin 'vindicare' meaning 'to claim' or 'to avenge'.

'avenge' entered Middle English from Old French 'avengier'; the present-participle/adjectival form 'avenging' was formed with -ing, and the adverbial form 'avengingly' was later formed by adding -ly to 'avenging'.

Initially related to the Latin sense of 'vindicare' meaning 'to claim, assert, or vindicate'; over time in English it came to mean specifically 'to inflict punishment in return' (to take vengeance), and the adverb came to mean 'in a vengeful manner.'