undersibling 

A gender neutral term for a niece or nephew.

My sister has a boy and a girl, I have two undersiblings.

undersibling by Smorke February 15, 2022

underbling

uterine jewelry, better known as an intrauterine contraceptive device (iud)

I'm going to the gynecologist today to get my underbling inspected.

underbling by gefion October 18, 2010