Supposedly Supposably Is Merely Supposition
Okay, I’ll keep this one short and sweet too: Supposably is not a word. Never has been, probably never will be. Sure, supposable is a word, as is its root, suppose. The kicker, though, is that the word most people are shooting for – supposedly, referring to “what is generally assumed or believed,” according to OED – when they say supposably is actually a word, while the latter bastard child of poor grammar isn’t.
The next time you’re about to whip out supposably, don’t. It doesn’t exist.