From what I can tell 'Daddy' is from the Daddy Dom/Little Girl Sub lifestyle. A woman who gains emotional/psychological benefit from regressing to a younger state and a Daddy dom who plays a parenting role. The Lg will have stuffed animals, maybe do colouring, wear young girl style clothing and throw tantrums when it's time to do boring stuff. Not all of the relationships have a sexual element though and many have difficulty understanding them and are frequently not understood. I struggle with understanding it completely. However if everyone is an adult and capable of giving informed consent then it's not a problem. There can be Mommy/Little Boy relationships too.
The term 'Daddy' then seems to have become more mainstream and exclusively sexual as a way for younger women to refer to older men they found attractive.
This article a couple of years old but interesting.
https://www.washingtonpost.com/news/...ting-about-it/