Supposably, is a word, although it doesn't mean what you think it means, and it's definitely not interchangeable with supposedly. It's an adverb derived from supposable, which means it's something you find conceivable. An example is when you begin a sentence with "for the sake of argument ..." Supposedly, on the other hand, is something you assume to be the case, although you have no specific evidence to support it. For example, supposedly, the concert tickets will be available before the show.
Can You Guess What These Long German Words Mean?
6 Minute Quiz 6 Min