What does it mean?
Sitzfleisch has two meanings. The first is that it is plainly another word for your buttocks, or Hinterteil.
The second meaning is difficult to translate into English: it's about one’s ability to sit patiently for a long period of time, a sort of sitting-stamina or sitting patience, but only in the right context.
If someone were to say: “Ich habe kein Sitzfleisch”, they don’t mean that they don’t have buttocks, they mean they don’t have the ability to sit still for a long time. But should they say something along the lines of “Au! Mein Sitzfleisch!” (“Ow! My ass!”) it has a slightly different connotation.
This meaning is also the same as the second meaning of another similar German word, namely (das) Sitzleder (which translates literally to “fitting leather”).
What are it’s origins?
The word Sitzfleisch consists of the words sitz (from the verb sitzen, meaning to sit) and Fleisch (meaning meat). So literally, your Sitzfleisch is your seat-meat, which already alludes to it’s primary meaning.
Its second meaning as sitting-stamina comes from the notion during the 60s that your presence of your backside is synonymous with having the ability to sit for a long time, which led to the creation of the phrase “kein Sitzfleisch haben” (meaning to not be able to sit for long). This essentially birthed the word`s second meaning as one’s ability to sit for a long time.
How is it used?
Sitzfleisch is usually used when complaining about sitting all day at work, or reminiscing about a 14-hour-long flight. It is also, of course, used if for some reason anybody would want to flex their knowledge of alternative German words for their rear end in a social situation.
Examples of ways to use it:
Wenn Sie in einem Flug nach Laos fliegen möchten, brauchen sie Sitzfleisch.
If you want to fly to Laos in one flight, you need sitting-stamina.
Ich möchte lieber nicht in einem Büro arbeiten: ich habe gar kein Sitzfleisch
I would rather not work in an office: I have absolutely no sitting-stamina.
Du bist nach nur zehn minuten gegangen? Na, du hast ja gar kein Sitzfleisch.
You left after only 10 minutes? Well, you have you don’t any sitting-patience.