ljin is a tough word to define, its more of a concept.
It used to be used exclusively for gangsters mobsters and meant "first team" aka the gang's best fighter. Now, it is used more frequently to refer to a school’s “bad” kids — the ones that are constantly disrupting class, getting in trouble, ditching school, smoking, drinking, etc.
We see them a lot in dramas.
Despite the negativity associated with the word, iljin are actually really proud of their title. Because of this there are even internet cafes and message boards where other iljin can talk and plan events!
These events, or parties, unfortunately, are basically where underage students engage in a whole bunch of behaviour that isn't super awesome for them... sexy dance competitions, make-out and groping competitions, and competitions that involve stripping down and imitating sex for the grand prize of a couple of packs of cigarettes.
Why is this related to Kpop? Because the netizens, of course, dug up some idols' pasts and found a few to be involved in that scene.
The most famous is T-ara's Hyomin....netizens found pretty concrete evidence that Hyomin was a self-proclaimed iljin (her name, photo, and ID were found on a list of attendees for an iljin party on a community message board). Efforts to trace her ID to an email account produced an email address that is identical to her Twitter handle.
Recently Soyou from Sistar was accused of being Iljin...but why?
Because she has a hairdresser's license.
She mentioned it on a reality show and netizens BLEW UP trying to prove she was a bully in high school.
Well, because Iljin are the bullies and delinquents of school, they usually drop out and can't get jobs that require a college or high school diploma.
That means they usually work jobs like being a hairdresser (you don't need to go to college, just a hairdresser school that anyone can apply to!) and THAT'S why people look down on hairdressers in Korea.
There is a huge stereotype towards people with those kinds of jobs (jobs that just need certification, not a degree) and the older generation often looks down on any young person in that career.
You can learn more about this here.