Webster's New World is a little vaguer, noting "supposably" only as a "related form" of "supposed." However, because WNW has a separate listing for "supposedly" and none for "supposably," there's no doubt which form they prefer.
So, what, exactly is the difference between the adverb form of "supposable" and the adverb form of "supposed"? In their adjective forms, they're clearly different. But as sentence adverbs, the distinction blurs.
I'm going to have to think on this one for a long, long time (after I stop retching, of course). Until then, I'll file this under "things I should have looked up a long time ago."
(Some find this distasteful, and I'm with them. But those who refuse to accept it need understand how words are formed and how dictionaries work. Dictionaries' job is not to tell us how we should use words. It's merely to document how we do use words. Then, once they do, we acquiesce to their wisdom and begin to accept these words as right. This is how baby words are born. The words we use and love today were once abominations. The word "ability" was once every bit as reviled as "supposably." Word creation is always an icky transition. But to resist it is to deny history. Anyone who uses "you" instead of "thee" and "thou" has already embraced the process. )
That said, allow me to add: "Supposably"? Yuck!
Causes June Casagrande Supports
Planned Parenthood, ClimateCrisis.net, the Richard Dawkins Foundation, Pet Orphans of Southern California, KIVA