Does saying "私は次男坊です" in self-intro sound humorous? why?

Question

I had a list of items to mention during extended 自己紹介 (like during a lunch or 飲み会). With those whom I also hoped to have friendship-like connection with, I'd also mention "私は次男坊です。" 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.

"私は次男坊です。" said alone, in a self-intro conversation, sounds funny, right? why? Would "私は長男です。" sound pretentious?

Or, I could I remembering incorrectly. I also used "次男坊ですから、許してください。" 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 卑下 funny, and there is nothing funny about simply saying "私は次男坊です". Or maybe my overall inability to speak Japanese funny. I hope you can help me remember.

Answer 1

You could have said 兄が1人います, which is fairly neutral. Or 私は次男です should have been better than 私は次男坊です at least a little.

次男坊 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 長男/次男 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, 私は次男坊です 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.