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The first time Thor had called Heimdall ‘dad’ was when he was little kid. He was eight and Loki had just stabbed him. Odin told him to see a healer and that he would be okay. So Thor had walked to the infirmary by himself. Loki was off being annoying somewhere else. Frigga was probably making sure that Loki was okay despite the fact that Thor was the one stabbed. And so Thor was left by himself.
He entered the infirmary and heard a gasp. The nurse, Eir, rushed up to him.
“Wow. I mean Heimdall told me what happened but I wasn’t you to actually have a stab hound!” she said, guiding him towards a chair.
“Heimdall? Why was he here? Is he okay?”
“Oh sure, he’s fine. He was just giving me a heads up about your condition. Wanted to make sure we’d be ready for you, I guess.” She healed his wound quickly, it wasn’t a difficult feat on Asgard. “There you go, kiddo. Good as new!” Thor thanked her and ran out of the room. When he reached the Bifrost, he jumped up on the dais.
“Hey, Heimdall. You saw I was stabbed?”
“Yes. I am happy to see you had a fast recovery.”
“Why didn’t you stay in the infirmary?” Thor asked, somewhat hurt that he had left before he arrived.
“I-” The dinner worn blew then, calling Thor back to the palace.
“I should go but thanks for looking out for me! Bye, Dad!” he called as he ran back the way he had come. He didn’t even realize what he had said.
The next time Thor called Heimdall his dad was a year later. Another kid had invited him to his house and Thor said he had to check with his dad. He hadn’t understood when the kid told him he would wait outside the throne room. He had planned to
bring Heimdall lunch at the Bifrost and wanted to make sure it was okay if he bailed. Odin wasn’t a dad to him, he was a father. Thor learned the difference.
After that, Thor called Heimdall dad often. They would share a meal or see each other in passing and he did it without thinking. He would refer to Heimdall as dad while talking to his friends, but he made sure to never do it around family. Thor loved
Heimdall and Heimdall loved him. He barely felt the ache of disappointment when Odin ignored him, he had a dad who cared. And that was good enough for him.
The last time Thor called Heimdall dad was when Thanos arrived. He cried for him. He mourned the loss of of his dad in a way that he hadn’t mourned for Odin. The ache was back in his chest, and no one could comfort him. Thanos would pay for what he had done.