Can you put a clearer pic?
If we imagine ourselves in the scene in which the witness speaks with a firm and confident voice saying "that is the man I saw", the most probable thing is that it convinces us because she is very sure of herself, the witness does not doubt it for a moment, she/he is stating it.
The word <u>averred</u> can be replaced by <u>stated</u> since they have the same meaning. Other synonims can be claim, declare or affirm.
The suffix "-fy" means cause or become. A verbal suffix meaning "to make" "cause to be" "render", "to become", "be made".The suffix was introduced into English in loan words from Old French, but also used the information of new words, usually on a Latin root. Hope this helps! If I'm incorrect please tell me as soon as possible! :-)