His profile photo shows a bald, monocled man, severe and unsmiling. His aphorisms, written in English, German, and "Denglish," are short, often ironic and always depressed.
In person, he's a pleasant man of 42, an academic with curly hair, dark-framed glasses, and a generous sense of humor.
Jarosinski developed his Internet persona as he felt himself at a dead-end. As an assistant professor of German at the University of Pennsylvania, he was supposed to write a book about transparency, the Weimar Republic, and contemporary German culture.
But as he tried to fill the pages, he was bored by his own book. He just couldn't stand the language, the long and complex sentences he was constructing. And he was falling into a real depression.
He let it out on Twitter, which he had recently learned about from a friend. He made up an avatar, a sour ball caricature of the philosopher Theodor W. Adorno. He found channeling such negativity great fun.
He greets his followers on Mondays with "The good news is that the coming week could be no worse than the last. The bad news is that it probably will be."
On Fridays, he releases them with "Weekend: The two days of the week when your alienation is all yours."
He plays with the thoughts of Hegel, Kant, and Freud, writes about capitalism: "Another beautiful day for wealth. Another inequitable day for its distribution." And he bites the hand that brought him some popularity: "Twitter's great if you're lonely. And intend to stay that way."
Over breakfast, he says that in Twitter's 140-character limit he has found his perfect medium. "I am not good in writing long books," he said. "But I love writing aphorisms."
His musings are followed across the world. A quarter of his readers are from the U.S., another quarter from Germany, and the others live in 150 countries.
Jarosinski's followers like his posts, he says, because his writings are a complex cocktail, "misanthropic yet romantic, authoritative yet ridiculous, principled yet darkly nihilistic."
And they like his perspective about Germany and German - "a language invented for philosophy but used to build automobiles."
Jarosinski was reared in a small Wisconsin town and started learning German at school. He stayed with the language, eventually earning his doctorate at the University of Wisconsin.
"The language never came easy to me," he said. But living in Germany for a time in his 20s, he eventually caught on. And he grew intrigued by reading the work of German philosophers of the 19th and 20th centuries. His pointed tweets about philosophical theories stand out in the often thin air of the Twitterverse.
"Most people have heard at school about Kafka or Nietzsche. You don't have to have read them to understand my tweets," Jarosinski says. "But one should go for playful contradiction and intelligent absurdity. The pleasure should be in the thinking."
The ability to write a memorable tweet is a skill one must develop, he says. "It is a craft. It needs exercising."
Sometimes the ideas come easily to him. While he's playing basketball in the morning in his neighborhood, a thought might arise and he'll rush to his mobile phone. Other tweets, though brief, take hours or even days to blossom into pessimistic pronouncements.
Now that he's written more than 33,000 missives, he feels he's getting better.
Good enough to pen a column of Twitter-length jokes for the prestigious German weekly newspaper Die Zeit.
He also publishes longer pieces in other German newspapers. He is building a blog. And his negativism has spawned an industry: there are NeinQuarterly T-shirts, mugs, and flasks. A book will be published in Germany and the Netherlands, he is still looking for a publisher in the U.S.
But Jarosinski is optimistic. He has left his academic job and plans to travel across Germany for a couple of months, then move to New York.
There, he wants to concentrate on what he thinks he is best at: sharing more grumpy, ironic, and smart aphorisms. His readers will appreciate that.