I hesitate to list this one because I don't know whose ox I am about to gore: "supposably".

Right off the top of my head I can name three (THREE!) well-educated, bright people I know that use that word.

Does it come from 'back in the day', like, during the time of Ben Franklin, the 1700s? That's what it sounds like to me. I have never commented on it to any speaker but I don't understand wherever they got it.