Sitzfleisch: The ability to endure or persist in an endeavor through sedentary determination. A borrowing from German, it literally means "sit-flesh"; a comparable English idiom might be "chair glue."
Sitzfleisch: a term used in chess to indicate winning by use of the glutei muscles--the habit of remaining stolid in one's seat hour by hour, making moves that are sound but uninspired, until one's opponent blunders through boredom. — Frank Vigor Morley, "My One Contribution to Chess", Chess Notes, Faber & Faber (1947). (Source: Wiktionary.)
I worked in the Hallmark public relations department for a man named Conrad Knickerbocker, the public relations manager, who had already begun publishing book reviews and fiction. After I got to know Knick a little, I asked him timidly how you become a writer. ... He said, "Rhodes, you apply ass to chair." I call that solid-gold advice the Knickerbocker Rule1. — Richard Rhodes, How to Write: Advice and Reflections (1995).
Other sitz- words that have been adopted into English include sitz bath (a shallow bath taken in a sitting position, with only the hips and pelvic region immersed); sitzkrieg (warfare characterized by lack of aggression or progress; the opposite of blitzkrieg); and sitzmark (the impression made in snow by a skier falling backward, a k a butt-plant).
Image from Zazzle.
1 I never knew Knickerbocker, so I call it sitzfleisch.