Reading: nomi no fuufu
Meaning: a couple in which the wife is bigger than the husband
Literally: a flea couple
When I first read this expression I knew why it was so immediately, but still thought it's amusing that this expression exists since I haven't heard something similar in English (or Hebrew).
The reasoning behind this is that in nearly all flea species, the female is larger than the male.
There you go, a nice expression and a piece of trivia, all in one!