質問: I'm struggling to understand what is being said in this line. To provide some context the person was discussing with a girl about his muscles and let her touch them. She then asked if he wanted to touch hers, and he declined (because of his conscience and the fact that she is a naïve sheltered girl). 「私のは......ぷにぷにですね......」 自分のお腹を触ってしょんぼりしていた。 「触ってみますか?」 「いや......自慢の腹筋になってから触らせてもらおう」 「ですよね......」 むしろぷにぷにのお腹を触らせていただきたいが、これ以上は良心の呵責というやつが。 **太ももを鍛えたなんたらいうシナリオが頭をよぎった**。 「体を鍛えるつもりなら、食事にも気を付けないといけないな」 「食べてこそ、血となり、肉となり、骨となる。しっかり食べないと大きくなれないんだぞ」 In the bolded line I am unsure what he saying. 頭をよぎる is used to say that something crossed one's mind. In this case 太ももを鍛えたなんたらいうシナリオ. 太ももを鍛える simply means to work out / train one's legs. What is confusing me is what なんたらいうシナリオ means and who 太ももを鍛えた is referring to.
回答: This sentence has なんたら, a placeholder similar to "you-know-what", "blah blah" or "whatchamacallit". See: The phrase うんやらかんやら? and What does なんたらという mean? 太ももを鍛えた modifies なんたら as a relative clause. As a whole, this appears to be a reference to a certain episode where someone worked out one's legs. This person used なんたら because he expected the reader could understand what he means only by saying 太ももを鍛えた. See this for a similar example. However, I am not aware of any well-known episode, title or cliche related to 太ももを鍛えた, and a quick google search gave nothing. Maybe this is a reference to some little-known net meme, or an episode understood only by the readers of this title. (The sentence usually has to be 太ももを鍛えたなんたら**と**いうシナリオ. It's probably a typo, but this と is occasionally omitted in casual speech or in some dialects.)