Running slowly without doing much work, like when a car engine runs gently at a stoplight; or spending time doing nothing in particular.
From Old English 'idel' (empty, useless), which is related to German 'eitel' (vain). The meaning evolved to describe both engines running at minimum speed and people wasting time.
Car engines were designed with an 'idle' setting in the 1920s specifically to prevent stalling, but the word perfectly described what the engine was 'doing'—essentially nothing useful.
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