Why was this closed, in view of claims to 'please include the research you've done...'?
As stated in my OP, I consulted Etymonline, OED, and a logic textbook.
Update on 2015 May 6: I thank the five users who generously reopened my question. Maybe no reply is needed to this question then, but I shall leave this for the benefit of others
(but please feel free to advise otherwise).