1. fail to increase; remain static: "their share of the vote has flatlined at about 3%"
2. (of a person) die: informal"they injected themselves with a deadly drug and flatlined"
▪ (of a project or undertaking) fail:"the film has flatlined"
Word Origin1970s: from flat1 + line1. flatline (sense 2) is a reference to the continuous straight line displayed on a heart monitor, indicating death.