We call my grandpa on my mom's side "Dud" bc my uncle had a different dad that died when he was really young and when he was like 6 they were discussing what he should call my grandpa bc he knew he wasn't his real dad but he didn't wanna just call him by his first name when the other kids didn't and my grandpa told the 6 year old he could call him whatever he wanted so my uncle decided on "dud" bc that was "kinda close but not really dad" and then everyone in his life still calls him that to this day