A mindless person or someone with no personality; gives blank stares, especially when presented with ideas or facts that should arouse some sort of reaction in a sentient being. Basically brain dead or mentally challenged. It would be improper to use this against someone who has a real diagnosed disability of course. More like, if you meet someone in a professional capacity whose title suggests they are intelligent, but really they just stand around looking dumb.
"I told Guido that the server was down and he said 'So...?'. Is that guy a fucking flatliner?"
by Dick Asplundher May 08, 2009

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Top Definition
Someone severely lacking in personality; a person who kills any festive mood with their utter dullness
Dottie was having a good time, until she got stuck with that flatliner.
by D*Tice June 08, 2003

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someone who is boring, unenergentic, tired, sleepy.
Dont be a flatliner! Here's a beer, c'mon lets get drunk!
by DanteM August 03, 2005

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A person whos eyebrows lack an arch.
Gentleman: Hon, you should really pluck your eyebrows, you look like a total flatliner.
Woman: Thanks, Hon.
by LadyGagum November 26, 2009

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A person who has died, referring to the heart monitor "flat lining", as can be seen in any film featuring an emergency room death.
When Ray makes love to a women, she may say, "it was like sex with a flat liner!"
by Mark J. February 23, 2006

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A Gun Used To End Someones Life (The Name Is Self Explanatory)
"Don't Make Me Run Up On Ya With That Flat Liner"
"Yo This Nigga Needs To Die, Ima Hit Him With That Flat Liner"
by HillByNature December 30, 2012

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An attempted one liner that is not funny at all.
*Kid sees another kid hanging from monkey bars*

"Hey, how's it hanging?"

*crickets*

"That's the definition of a flat liner."
by ADR767 September 09, 2013

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