Some friends and I recently debated whether "supposably" is a word. Turns out, it's a word...but it isn't a synonym for "supposedly" in the way that so many toddlers use it. According to Webster's:
Supposable \s&-'pO-z&-b&l\, adj.
Capable of being supposed: CONCEIVABLE
sup·pos·ably \-blE\ adv.