Supposedly:

sup·posed adj.

1. Presumed to be true or real without conclusive evidence.
2. Intended: medication that is supposed to relieve pain.
3. 1. Required: He is supposed to go to the store.
   2. Permitted: We are not supposed to smoke here.
   3. Firmly believed; expected: You’re supposed to be my friend.

sup·pos’ed·ly (-pō’zĭd-lē) adv.

Supposably:

sup·pos·a·ble adj.
That can be supposed or conjectured: a supposable outcome.
sup·pos’a·bly adv.

In looking at these words, they look pretty similar. However, a little deeper look shows that the word “supposably” is pretty much an American English word. Meaning only Americans use it. It doesn’t “exist” in other English-speaking regions. Not only that, it just sounds kinda dumb. Like a 5-year-old trying to say “supposedly.” There is only one instance where “supposably” is to be used (and you can substitute it for “supposedly” and still be correct), which I can’t really come up with an example. Mainly I don’t want to, as the word is pointless. Heck, even people at WSU know better: http://www.wsu.edu/~brians/errors/supposably.html.

Luckily, I know few people who use this word “supposably” and just assumed they were kind of an idiot for mispronouncing it. Not like I’ve ever mispronounced anything, like on an instructional video for software developers. No, never.

So if you use “supposably” come back to the proper side of English and stop using it.