Spouses sharing a spouse, as two husbands of a wife, or two wives of a husband.
Today's word, like yesterday's, is not something that we typically practice today (not in Hawai'i, at least!). Yesterday's word, po'olua, referred to a child who had two fathers. Today's word, punalua, refers to the relationship between two husbands or two wives who share the same spouse.
Hawaiians, back in the pre-missionary days (and even in the early years after their arrival in 1820) practiced polygamy. A husband could have many wives and a wife could have many husbands. This was more prevalent among the ali'i (chiefly) class. Kamehameha had many wives. The two most well known are Ka'ahumanu and Keōpūolani. They were punalua to each other since they both shared the same husband. Polygamy wasn't practiced for the fun of it. It was more for social ranking (by having children with Keōpūolani, a very sacred ali'i, Kamehameha I assured the continuation of the monarchy by his children) and economics. A wife may ask her husband to take her widowed sister as a wife so that there would be a man and a family to care for her.
For whatever reason, it was uncommon for punalua to be jealous of one another. There are some stories/legends that tell of a jealous punalua and doom being the outcome of a jealous rage.
In modern day Hawaiʻi, I know of one punalua relationship (this was years ago) and I am wondering about another. I am sure it happens here in our own islands, along with all of the other "out of the norm" relationships out there. To each his (or her) own. I do not sit in judgment, lest I be judged.
'O Ka'ahumanu ka punalua o Keōpūolani - Ka'ahumanu was the punalua of Keōpūolani.
'O wai kona punalua? - Who is his punalua?