Tony calls his daughter Maguna a couple of times in the movie. (Or at least uses that word around her. I don't know if that's just a nickname or has a deeper context. Any search on Google yields no proper results regarding "Maguna" in relation to the MCU.)
He also calls her by her full name, Morgan H. Stark. Searching for the name in the comics, I came across this. Apparently, Morgan is a different person there, an evil cousin of Tony's. I haven't read any comics so I don't know much about the characters there.
There was also a line in Avengers: Infinity War where Tony says to Pepper that he wants to name their future child by the name Morgan and that's exactly what they did so where does Maguna come in?
I'm unable to find the Endgame script or images of the scenes where Maguna is mentioned, apologies.
Edit: I know most think it's just a nickname Tony addresses his daughter by and I quite agree but it seems unusual with an interesting story to it, which wasn't mentioned in the movie. I was hoping someone who has good knowledge of the comics side of things might have come across it. If this indeed is the first mention of Maguna in the movie and comic universes, I'm ready to accept the answer mentioning it's a nickname.