It's the same word as supposedly, but we totally pronounce it wrong. I've said this word wrong for as long as I could remember, until someone was like, "why are you saying 'supposedly' like that? (Actually I still pronounce it wrong).
Examples of How to Use the Word:
"Suposably, Caro got her tetas done. Bro...they are so beautiful!"
"Bro, this guy told me I say 'supposably' wrong. Like, what is his life?!"
"Okay, so, Ashley supposably is dating William Levy. I'm so jelly!"