Does saying "私は次男坊です" in self-intro sound humorous? why? I had a list of items to mention during extended (like during a lunch or ). With those whom I also hoped to have friendship-like connection with, I'd also mention "" This allowed me to talk about my siblings which I was happy to do. And, I think it made the native speaker smile or laugh a little. "" said alone, in a self-intro conversation, sounds funny, right? why? Would "" sound pretentious? Or, I could I remembering incorrectly. I also used "" as a way to be self-depricating. In Japanese culture, the 1st born son is king, the second born is trash, so I would (to be funny) excuse my failures to being born the second son (nothing I could control). Maybe they found that funny, and there is nothing funny about simply saying "". Or maybe my overall inability to speak Japanese funny. I hope you can help me remember.

You could have said 1, which is fairly neutral. Or should have been better than at least a little.

is indeed a nuanced word with a certain connotation related to the **old** Japanese view about family/business inheritance (see this). However, in ordinary business settings in modern Westernized companies, almost no one cares about one's siblings, and the / distinction you described is not a thing. It does not work as an explanation of an error even if a joke is intended.

In your situation, is strange even if said by a native speaker, but it might have sounded funnier because it was said by a foreigner. It implies you know a difficult word but have some misunderstanding about it.