If you wanted to be literal it would be sit flesh, perhaps sitting muscles, like working memory and in the broadest sense it might mean composure, patience, ded

If you wanted to be literal it would be sit flesh, perhaps sitting muscles, like working memory and in the broadest sense it might mean composure, patience, dedication in a restricted sense, or in a literally broad sense it means the buttox. Probably not a fat ass gotten from sitting around and neglecting exercise but eating like a weight lifter, although that was my first association in context ... compare "Arsch in der Hose haben" - "to be a hard/bad-ass", referring to courage, abillity and composure more generally, not too different from the massive balls meme. Great story bbc, very tabloid vulgar after all.