Roseanne Buckley’s 18-year-old son had always been a mild-mannered homebody and excellent student. But in March of his senior year in high school, she said the young man’s mood changed.

“He was doing things out of character or without mentioning it first,” said Buckley, who lives in Medford, New Jersey.

One time, he let a friend pierce his ear, and her son suddenly developed a snarkier and more defiant attitude toward her, she said.

Buckley, 49, learned the social media slang for this sudden defiance, which is called “soiling the nest,” and said she felt that was happening with him.

“It just felt like out of nowhere he was acting very, ‘I can do what I want because I’m 18,’” she said. “He was a completely different kid.”

Stretching their wings

Soiling the nest refers to a phase when teenagers start being more difficult at home as they’re transitioning and getting ready to go off to college, said clinical psychologist Regine Galanti, founder of the practice Long Island Behavioral Psychology on Long Island, New York, and author of the young adult nonfiction book “Anxiety Relief for Teens.”

This behavior can show up in different ways.

“It may look like picking fights with parents, talking back, leaving messes, and generally putting stress on their interpersonal relationships with their parents,” Galanti said.

And it’s a completely normal phase of growing up.

For teens graduating high school, the main psychological goal is to “develop and consolidate their own identities,” Galanti said. Since doing so can be difficult in the shadow of parents and family, some teens may assert their independence and push away from their families at this time.

Creating this space between themselves and their parents makes the transition to this new independence phase easier for some teens, Galanti said. It can help if parents expect it, even though it won’t always happen.

“Recognize that, as a parent, this is not about you,” she said. “This is about your child having difficulty figuring out how they’re going to manage the next phase of their lives. Pushing you away might make it easier for teens to launch themselves into independence.”

When your child becomes your roommate

Gabriella Neske, a mother from Tampa, Florida, said she experienced the phenomenon as her oldest daughter was getting ready to head to college. She would hide in her room, glued to her phone, acting “generally sullen,” said Neske, who is already experiencing similar behavior from her twin sons, currently high school juniors.

It felt like her kids were trying to transition into her roommates, Neske said, openly defying her when asked to do something and doing irritating things like leaving piles of wet laundry atop the dryer.

As a parent, Neske said she tries to give her kids the opportunity for independence while still keeping the guardrails up and letting them try to develop their own identities.

And that’s when things can get complicated.

“You still have to step up and try to parent that sullen little ball of emotion that doesn’t really have the skills to go out and be an adult in the world — yet,” Neske said.

How to soften the blow of a checked-out teen

Psychologist Lisa Damour, the author of “The Emotional Lives of Teenagers,” advises parents not to take it personally if their teenager is holding them at arm’s length. “Trust it will be one chapter in a long book of raising that child,” Damour said.

The months leading up to a child leaving for college are emotionally charged for all members of the family, and often, parents and their teens are in different places emotionally, she said.

While it’s not unusual for parents to want to savor every close moment at this time and impart wisdom, teach life lessons that haven’t yet been shared and commune in all sorts of ways, teenagers are often trying to spend every last minute with their high school friends whom they’ll soon be missing, Damour said, which can lead to a mismatch of priorities.

“I think that it can be helpful to remember it’s not personal, that’s it’s a natural part of development and that, in some ways, the opposite would be strange,” she said. “If a teenager getting ready to leave for college was suddenly nestling into the bosom of family life, that would be odd.

“And if we can appreciate that holding one’s family at arm’s length is part of a process of moving away, it can feel less personal and hopefully less painful.”

Keep in mind, too, that not everything teenagers are up to is entirely conscious, Damour added. “They’re not always deliberately choosing to be prickly,” she said. “We need to appreciate that, for most teenagers, leaving home is really hard, however excited they are for what’s ahead.”

Teenagers will sometimes be difficult because it’s easier to say goodbye if everyone is annoyed with each other, she said.

You might also find that suddenly, your teen is nowhere to be found. “Even when you finally run into them in the kitchen, they don’t have time for a conversation — even though you might be trying to nail down critical logistics of getting them off to college,” Damour said. It can be helpful to say something like: “I’m going to need 10 minutes to talk to you about your packing. When is a good time tomorrow?”

If you understand that your kid has a lot going on, “it can smooth interactions if we are willing to coordinate our calendar with theirs,” she said.

Parents should also notice how forlorn younger siblings are since it can be a tough transition for them when an older sibling moves out.

“Take very good care of yourself and get your emotional needs met elsewhere,” she said, “so if you have younger children you can tend to their feelings about their sibling moving out, too.”

As for Buckley, she said her son reverted to his normal self as his move-out day in mid-August approached.

“My kids’ generation seems to have parents who do everything for them,” she said. “It must be a scary feeling to be going far away and suddenly having to do everything yourself.”

Once their independence has been established, Damour finds the new phase often ushers in a warmer relationship.

“It may take a full freshman (in college) year, but teenagers tend to be warmer, friendlier and in fact quite wonderful to interact with in their increasingly adult form,” she said.

Florida-based writer Terry Ward lives in Tampa with her two young children, who are still squarely ensconced in the nest.