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2.
Someone who pretends to be happy when they're not, referring to the confused bisexual narrator of The Killers' album Hot Fuss.
But it's just the price I pay
Destiny is calling me
Open up my eager eyes
'Cause I'm Mr Brightside
by Zythe November 02, 2005
 
1.
In a reference to the song "Mr. Brightside" by the Killers, a brightside is someone who is jealous of his/her ex's new beau.

"He takes off her dress now
Let me go
And I just can't look
It's killing me
And taking control"
"Destiny is calling me
Open up my eager eyes
I'm Mr. Brightside"
by Catmka March 07, 2005