flatline

The iPhone is by far Apple's most important product, accounting for nearly two-thirds of the company's sales last year. But iPhone sales have flatlined since 2015, frustrating millions of investors in one of the market's most widely held stocks

flat-lining
having no heart rate. basically dying

flatline

Pronunciation: /ˈflatlʌɪn /

VERB

[NO OBJECT]

1informal (Of a person) die:they injected themselves with a deadly drug and flatlined

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1.1(Of a project or undertaking) fail:her career has flatlined about three timesalready

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2Fail to increase; remain static:their share of the vote has flatlined at about 3%

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Origin

1980s: from flat¹ + line¹ (with reference to the continuous straight line displayed on a heart monitor, indicating death).