Well, サザエさん[Sazae-san] is, if nothing else, a long-running TV anime series though it’s not very popular among anime fans. Yes, this anime is for non-otaku folks as well as ちびまる子ちゃん[Chibimaruko-chan]. As it happens, these two anime are broadcasted in a row, from six to seven p.m. on Sunday, when most families would have dinner. Both of them are midiocre sitcom comedies if you ask me. Forgive me, but I do not like this kind of thing because it’s too good to be true.
Anyway get to the point, Sazae doubles up with her parents at her family home. So does her husband, マスオ[Masuo]. See what I mean? He lives with her whole family members; her parents and two younger siblings of hers. In other words, he is always surrounded by them at home…he has few private hours so badly. He can’t relax all the time even though he seems to be happy in the anime. I hate to tell you, but I don’t think their house is so enough to accomodate so many people, though.
So this term, マスオさん means a husband(or an engaged man) who lives with his wife’s family at her home. Know this term aims a satire at such a person. Yes, I think this is a slang, and even non-otaku folks know this because this anime is for such general people.
By the way, I know at least two マスオさんs who happen to be foreigners. One is English and the other is French. Both of them have a Japanese girlfriend. I have never heard any complaints or whining from the two guys about being マスオさん. For non-Japanese people, being マスオさん is not stressful?