'flatline' originates from the combination of 'flat' and 'line', where 'flat' meant 'level or even' and 'line' referred to a 'continuous mark'.

'flatline' evolved from the medical term used to describe a lack of heartbeat on an ECG monitor, eventually becoming a metaphor for lack of activity or growth.

Initially, it meant 'no heartbeat', but over time it evolved into its current metaphorical meaning of 'no activity or growth'.