An atheistian (atheist + Christian) is a stereotypical militant atheist who essentially treats atheism like a religion.
While atheistians will of course deny that atheism "is just another religion", they have a set of unwritten rules which they expects all members of the "atheist community" to follow
Basically a "true atheist" (according to atheistians) is a person who is one or more of the following - a communist, socialist, liberal activist, radical feminist, tree hugging environmentalist, welfare junkie, emo kid, goth, computer geek, fat fuck, virgin, homosexual, weeaboo, D&D nerd, social reject, aspie, basement dweller, or just a straight up whiney-ass loser in general who bitches about "atheist persecution" but is less tolerant than a lot of fundies.
Any atheist who doesn't fit this stereotype (ex. supporting gun ownership rights, not voting for the Socialist Party, playing sports, having a sex life, enjoying mainstream movies and music, not sporting a neckbeard, etc) is considered on par with a religious heretic by the atheistians and despised even more than Christians.
Atheist dude: I'm a big fan of Bill Maher. Anyone else here watch his show?
Atheistian 13 year old virgin: What??? Bill Maher??!? You're a fucking ignorant moron to watch that douchebag! That guy spouts anti-vaccine propaganda and he's a horrible representative for the atheist community, and you're as ignorant as he is for watching his pathetic show. STFU moron.
Atheist dude: ...uh hey dude, Bill Maher's actually a deist, not an atheist.
Atheistian 13 year old virgin: ..what what? Uh nevermind I don't care. You're a moron and you'd make a great creationist considering how fucking stupid you are. Quit giving atheists a bad name, you fucking moron. I'm going to ignore you from now on, you fucking idiot. Bye.