In the song “Daughter,” sung by Loudon Wainwright III in the 2007 movie Knocked Up, there’s a line that goes:

“That’s my daughter in the water, everything she knows I taught her/ Everything she knows.”

Now, that’s an arrogant, narcissistic dad lyric, I acknowledge. But when my daughter was 3, and I was a single father, this pretty much summed up how I used to think. So outsize was my paternal influence, I believed, that the predominant contents of her brain had to have originated with me (and, of course, with her mom).

But now my girl is 15, and the volume of what she knows that can be directly attributed to me roughly boils down to: The best bagels come from Brooklyn, and Eli Manning is a better quarterback than people say.

My daughter is assimilating knowledge through her own filter. And, more and more, dad’s savvy means less and less. In a few short years, I’ve gone from deity to doofus.

Usually, to learn what’s on my daughter’s mind, I have to beseech her to toss me a thought morsel, which is about as easy as asking Robert Mueller what he’s up to these days. Still, it’s not too hard to figure out at least one thing my kid’s preoccupied with this year:

The South Korean boy band known as BTS.

‘Hysterical rapture’

I felt woefully uninformed about the K-pop (Korean pop music) phenomenon, fostered by a massive, big-bucks entertainment machine. It took the sight of my girl plastering her bedroom walls with posters of cute and earnest Asian men in their 20s for me to get the picture.

Combining hip-hop with pop harmonies and athletic choreography, the seven members of BTS — RM, Jungkook, V, Jimin, J-Hope, Suga, and Jin — make up the most popular boy band in the world. Their “Love Yourself: Tear” album debuted at No. 1 last May on the Billboard 200, the first and only Korean act to do so. The fellas, who got together in 2013, sing mostly in Korean and Japanese, with occasional English words. I must admit they’re talented.

BTS (which stands for, depending on who’s interpreting, Bangtan Boys, or Beyond the Scene) is loved, the New Yorker says, with “hysterical rapture" by its fans, known as ARMY, or Adorable Representative M.C. for Youth. Writing in the newsletter Odyssey, one young woman called her favorite band member, V, her “Lord and Savior,” adding, “Like when he bites his lip. I swear, I die.”

The boys “friggin’ LOVE their fans and treat them all like baby angels,” according to Buzzfeed. They openly discuss LGBTQ rights, mental health, and the pressure students face to succeed — all taboo subjects in South Korea, says Rolling Stone.

ARMY has a huge online presence. Bloomberg reports that BTS broke the world record with most Twitter engagements in 2017, meaning the total number of times a BTS-related tweet was retweeted or liked on Twitter reached 500 million — more than President Donald Trump (213 million), or singer Justin Bieber (22 million) combined.

Last September, BTS addressed the United Nations with a message of love. And last week, their concert film “BTS World Tour: Love Yourself in Seoul” was seen in 3,800 theaters in 95 countries, said to be the largest worldwide cinematic release of a single event in history, according to Billboard.

I took my girl and a pal of hers to a Delaware movie house to see the film. She didn’t yell or swoon; she simply watched intently. I’d love to take her to a concert, but I couldn’t afford the band’s sold-out Citi Field performance in Queens last summer, with tickets around $500 to $600 each. Still, we had a very BTS Christmas, with much merch under the tree.

‘Crazy, safe’

I’ve seen fervent fan love before. The Beatles, David Cassidy, the Monkees, Michael Jackson — they all solicited intense reaction from girls and women I knew growing up. Not all BTS fans are female, but so many on Twitter appear to be.

“It’s a group experience that gives girls permission to be really hyped up about something unattainable and idolized,” says Seattle psychologist Christine Nicholson, who specializes in the psychology of girls, and once screamed for Elvis back in the day. “You can let your feelings go in a crazy, safe way. It’s a high.”

The BTS experience shows how my family has come a long way from when my daughter used to ask me to play my guitar and sing Springsteen songs with her — a wistful memory for me, an unspeakable embarrassment for her.

Parents realize that they must learn their place, then shrink it — and shrink it again — so their kids can grow.

That’s why I yield to the Seoul singers with their winking radiance and trendy togs. Dockers dads like me have no style, anyway.

Like Jimin sings in “Serendipity”: “The whole world is different from yesterday.”

It’s about time I figured that out.