A person that claims they are always correct even when they are wrong and will continue to claim they are correct even after being proved wrong with hard evidence. Normally a sign of some forms of Autism hence the word used.
Steve: "Dave's so autismistic. He's always claiming that Jack Black is Hugh Jackman's son and that he was forcefully made to change his name because the embarrassment caused by him being in 'The Muppets' film."
Bob: "What kind of bullshit fact is that. It doesn't make any sense at all, did you show him their birth-dates and how that would be impossible?"
Steve: "Yes but he claims that a time machine was used for it to be possible and I was stupid and incorrect for not thinking of that."
It's when your friend is the last one alive in rainbow 6 seige and he truly thinks he can pull the 1v4 clutch but you know he is autistic so therefore he is an autistimistic
I saw this funny looking kid at school and he was confident is asking this girl out I must tell you he is very autistimistic
when you're holding up your phone and making faces at it, as though you are taking a selfie, but you're really taking a picture of the person across from you or the wall or anything else that seems interesting but you don't want to be caught dead taking a picture of.
This action is often made more convincing by wiggling the eyebrows or opening the mouth, to pretend you're trying to get a Snapchat filter to work.