Before writing on this word, I decided to see what the on-line dictionaries had to say. Merriam-Webster said they had decided to import it from the Unabridged. Others just said it is a synonym of otherwise. Some fora (Lt. plural of forum) had comments that dismissed the word. Sorry folks, but it’s always been in my vocabulary, albeit otherwise is much more common. Sometimes I wonder where I came from. I grew up in Ohio and Arizona. How American is that? Oh, yeah, I was born in Kenya – I forgot.
My old Webster’s New Collegiate(1973) says it means, among other things, if the facts are or were different. The OED lists it forthrightly and cites an 1888 occurrence in the North American Review.