'avouchment' originates from Middle English, via Anglo-Norman and Old French (cf. 'avoucher'/'avoucher'), where the element 'a-' is a verbal prefix and the root is related to Old French forms of 'vouch' derived from Latin 'vocare' meaning 'to call'.

'avouchment' changed from Middle English forms such as 'avouchem(e)/avouchment' derived from Old French 'avoucher' and ultimately from Latin-rooted words; the Middle English verbal forms ('avouchen'/'avouchen') produced the noun form that became 'avouchment' in modern English.

Initially it carried the sense of 'to call forward or formally assert (something)' and 'to vouch for' (a verbal attestation); over time it settled into the noun sense 'a formal assertion or attestation' used as proof or declaration.