The word "suppositious" – that is, according to Google, which is trying very hard to be a know-it-all (in the very best sense) – means conjectural. There, that should satisfy ya.
If it doesn't, then search Google for the word. The most linked-to site – I guess that's what it means when someone's listed first in Google – is Worldwebonline.com. And the little squib that you can see when you do a search, before you really click the link, says, suppositious is an adjective meaning "Based primarily surmise rather than adequate evidence."
Well, I never! Are they implying that we're non-evidential. We're very evidential. In fact, we're so evidential, that our ceilings are about to crack. That's how evidential we are.
In any case, we try to hang a little supposition onto a great big hunk of fact. Or is it the other way round? In any case, you certainly would be silly to believe everything you read without running and checking wouldn't you? So if you jump to some conclusion as a result of something you read here, who's to blame? You, that's who!