ETYMOLOGY
apparently from affy (to trust in someone or something) + – ant + ‑ly
EXAMPLE
“…St. John saith expressely, Jesus did many things which are not written. Indeed we may chuse whether we will affiantly beleeve any thing that is not written; but to say positively, It never was, because it is not written, is unworthy Hieromes learning, and is elsewhere by him rejected, out of the humour of opposition. …”
From: The Acts and Monuments of the Church Before Christ Incarnate
By Richard Montagu, 1642
Lexicophilia 