a person sentenced to life in prison, or someone who spends their entire career devoted to one organization or cause.
From 'life' plus the agent suffix '-er' (one who does something). The prison sense emerged in early 20th-century slang; the career sense followed later.
The word has two completely different meanings depending on context—a prisoner with no release date, or a devoted military/career person. The etymology is identical because both meanings suggest being committed to something for the entire duration of your life, whether that's chosen commitment or forced confinement.
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