'avouch' originates from Anglo-French, specifically the word 'avouchier', where the prefix 'a-' (from Latin 'ad-') meant 'to/toward' and the root related to Old French terms meaning 'to declare' or 'to justify'.

'avouch' changed from Anglo-French 'avouchier' and Middle English 'avouchen' and eventually became the modern English word 'avouch'.

Initially, it meant 'to call in support or to justify' or 'to acknowledge as true', and over time it evolved into the current meaning of 'to assert, affirm, or vouch for'.