(Oni no nyoubou ni kijin;
“For an oni’s wife, a female oni”)
The woman who becomes an awful man’s wife will tend to be awful as well. A married couple will tend to resemble each other – even or especially the bad ones. The implication seems to be that any gentle soul would refuse to marry a cruel and violent person, so the cruel and the violent will tend to pair off.
We begin with the noun 鬼 (oni), a folkloric entity that may be rendered as anything from “demon” to “ogre”; its main characteristics are a lack of human compassion, and monstrous strength. Next is the associative particle の (no) in its possessive function, marking the noun 女房 (nyoubou), “wife.” And at the end of the phrase we have the compound noun 鬼神 (kishin), which in this case is probably best understood as an alternate way of saying oni. This is preceded by the particle に (ni), which indicates the position or location “to” which the 鬼神 does some action.
In this case, an expanded version of the phrase explains what that action is: なる (naru), “to become” proceed by a が (ga) which marks kijin as the subject doing the changing. In other words, “a 鬼神 [becomes] the wife of an 鬼.”
I’m tempted to launch into a mini-essay about how true this saying might actually be, but don’t really have the time or energy for that right now. Suffice to say that it’s presented here in the sense of a window into a past culture rather than an eternal, universal human truth.
鬼神 are ferocious, generally malevolent beings (originally, angry spirits of the dead). However, in 『玉勝間』 (Tamakatsuma) – the Edo-era text which this phrase comes from – 鬼神 is apparently simply used to mean a female oni. Note that while 鬼神 may often be read as kishin or even onigami, this saying seems to always read the second character as jin.
A variant phrase inverts the grammar by declaring that 鬼の女房に鬼神の亭主 (oni no nyoubou ni kijin no teishu), “A 鬼神 husband for an 鬼 wife.”
This is the を entry of the Osaka iroha karuta set.
(“Takeda wa koukou no toki, hidoi ijimekko datta. Daigaku mo sanrou shite youyaku ukatta no ni nyuugaku shite sugu chuutai shite gyangu ni kakawatta rashii. Oni no nyoubou ni kijin ga naru to iu kara, yappari juunen tatta kedo dousoukai de wa honnin ni mo, okusan ni mo aitakunai.”)
[“Takeda was a horrible bully in high school. And I hear that in college he failed the entrance exams three years in a row, then when he finally got accepted, he immediately dropped out and got involved in a gang. Yeah, ten years may have passed, but ‘an oni’s woman is another oni,’ so I don’t want to meet him or his wife at the school reunion.”]