Self-importance 

An exaggerated sense of one’s own value or importance. -Emeasoba George

Self-Importance is synonymous with arrogance.

Self-importance by Emeasoba George June 9, 2018

unwarranted self-importance 

A serious form of narcissism. To act as if one is an important person without actually doing anything worthwhile to prove it, thus making one look like a complete tool.

Sometimes abbreviated as USI.

Idiot: Hey guys! I just put up all these sweet pictures of myself on Facebook, and I've got this vlog that you've gotta check out. If you don't, you're clearly some kind of lesser being, 'cause everyone loves me for no reason!

Smart Person: Sounds like unwarranted self-importance to me.

unwarranted self-importance by IrishBarbarian January 23, 2011

Self-Important Sniff

A quick and audible nasal inhalation made after making a declarative statement about a topic that one's audience supposedly knows little and/or the sniffer considers himself/herself somewhat of an expert. The self-important sniff serves as punctuation to a statement that is typically unquestionable as the sniffer may attempt to pass opinion off as fact. It is typically used by hipsters, although it is also used mockingly, and to some comedic effect, on the Adam Carolla Podcast, as well as The Film Vault (podcast) by hosts Bald Bryan and Dick Anderson.

Bald Bryan: "Micheal Parks...is the defacto star (Self-important Sniff)"

Self-Important Sniff by Mjim#9 November 7, 2011

self-important 

a person who overestimates their own importance and exhibits arrogant or pompous behavior
aka lunchmeat

That self-important bitch just cut in front of us in line; she's too busy lunchmeating on her cell phone to notice anyone but herself.

self-important by andy M April 2, 2004

self important bitch

Basically a fucking girl who thinks she can do w/e she wants b/c she has nice tits and a nice ass and treats people like shit for her own satisfaction of feeling above others

AYYO go fuck yourself you self important bitch and get my attention normally

self important bitch by Trippinballs January 26, 2007

Self-Important Program 

Like computer programs that run automatically on your computer, whether you want them to run or not, because their programmers have decided they automatically should. Any person (or program) that takes over from a mind set of self-importance, whether someone has asked them to be the center of attention or not.

Ever since Amy came in here, and took over, things haven't been going well.
Yeah, she's really quite the self-important program isn 't she?

My computer takes a long time to reboot, it has lots of self-important programs running.

Self-Important Program by TranceMasterJ March 16, 2012