According to Zokugo-dict:
Masuo-san refers to the husband of Sazae-san in the popular anime "Sazae-san", Fuguta Masuo. Masao-san lives together with Sazae-san and their parents at their parent's home, and it's come to mean a person who lives at their wife's parents' home.
Furthermore, it's become used in a broader sense to refer to a son-in-law taken into a family (婿{むこ}養{よう}子{し}; in the Anime Masuo keeps his Fuguta name, so isn't a 婿養子.)
In particular, because in 1989 these kind of males increased, the expression "Masuo-san" became a trending phenomena.