"I Want Attention" Syndrome is a slang for a person with HPD (Histrionic personality disorder). This kind of person would have an irritating appearance. Seeking approval and craving attention from others. Leading to inappropriate behaviors, extreme sensitivity, emotionally dramatic, and others having factitious somatic symptoms; Mistaking or faking an illness, an most common example is depression.
This sighting is more common on the internet with social medias. Defining someone with 'I Want Attention Syndrome' is to poke fun at them for their behavior. It is usually a bait for that person to make them angry, or a definition for a "group of people." Effectively, it's just a one-liner to define someone on Twitter.
"Okay, Mr. "I Want Attention" Syndrome.