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!"
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"