Also see definition of "Avouch" in Word Study
Avouch
AVOUCH [isbe]
AVOUCH - a-vouch': In English Versions of the Bible only in Dt 26:17,18, in the sense of "to confess," "avow," "publicly and solemnly declare." The Hebrew form is likewise unique (Hiph. of 'amar).
Also see definition of "Avouch" in Word Study