verb
avouch
uh-VOWCH/əˈvaʊtʃ/
Declare confidently that something is true.
The craftsperson avouches that the repaired chair is sturdy enough to use.

Etymology
To “call to” meant summoning a witness or invoking an authority. That appeal to testimony underlies avouch’s sense of affirming a claim or guaranteeing its truth.
More examples
- I can avouch the truth of her account.
- He avouched his loyalty before the assembled council.
- The letter avouches the writer's confidence in the findings.
Forms
avouch · avouches · avouched · avouching