Who was Sirius Black's love interest

What is the meaning of Lily Luna Potter's middle name?

The meaning of her middle name is never given in the epilogue.

It isn't even claimed that her middle name is actually in honor of Luna Lovegood. However, it is a fair assumption that it does, as she is the only Luna mentioned so far. In addition, the epilogue marks the end of the Harry Potter series. There would be no reason to write to another Luna that we have never seen before, and if Lily were to have a middle name that is not related to anyone we know, she would have been given a name that was not shared by anyone who played a significant role in the story.

Although Luna was a good friend to both Harry and Ginny, she seems like an odd choice for naming one of her children after him. It also contributed to the war and I am not suggesting that it is unimportant or that it did not play a significant role in Harry's life.

But Harry can't possibly name a child after everyone he cared about. There are far too many people who have helped him for this to be possible. He had to either have a ridiculously large number of children or give them all long Dumbledore-style names to honor all who deserve it. There's Arthur Weasley, Neville Longbottom, Mad-Eye Moody, Professor McGonagall, Tonks, Cedric and Kingsley Shacklebolt on. It would make sense if you consciously or unconsciously narrowed down the list of name choices in some way.

Given that Harry and Ginny would have to carefully choose who to honor by name, Luna is an odd choice. She was their classmate and probably still alive when they named their daughter. If they haven't grown apart over the years, chances are they'll still see Luna and be in contact with her. Something similar could be said about Neville. He was strongly represented in both Harry and Ginny's lives and performed heroic deeds during the war. However, none of the Potter children has a name related to Neville. Which is understandable - the epilogue shows that the Potter family has stayed close to him and he leads a happy life as a professor of herbalism at Hogwarts. Both Harry and Ginny liked Luna, so it seems likely that they would have maintained some level of friendship with her - and if for some reason they hadn't and argued with them, they probably wouldn't want to call their child after her.

The great thing Harry's children's names have in common, aside from being the people who meant the most to him, is that their namesakes are all dead and died tragically as a result of the Wizarding War. This is most likely because Harry wanted to honor those who died in the war and gave their lives to help him win it. It's also a way to somehow manage the list of choices. It is also more practical to name children after a dead person than after a living one. Having two living people with the same name could be confusing, especially if Harry stayed in close contact with the original owner of the name - which he likely would do if he respected them enough to name a child after it.

The best theory I've heard is that it is also in honor of Lupine, but this has not been proven.

Harry's first son, James Sirius, was named after his father and godfather. Harry's second son, Albus Severus, was named after his late mentor and teacher who suffered and ultimately sacrificed his own life for Harry. Lily is the youngest Potter child and her first name is in honor of Harry's mother.

Maybe Harry wanted to honor Lupine with his child's name as well, but since she was a girl he couldn't exactly call her Lily Remus because Remus is obviously a boy's name. There is also no feminine form of Remus or Lupine. So if he wanted Lily's name to be his honor, he'd have to be a little more creative.

Luna sounds like Lupine and means moon. Lupine was a werewolf whose transformation was caused by the full moon, and Lupin's Marauder nickname was Moony. Both he and Ginny would have positive associations with the name due to Luna Lovegood as they were both good friends with her. These might not be enough to convince them to name their child just after Luna, but it could help encourage them to choose the name in honor of Lupine.

Unyielding

I mean the feminine form of lupine is lupa.

ibid

Rowling said on Twitter that Harry didn't name any children after Lupine.