To make something longer or to become longer in time or space.
From Old English 'lengten,' formed from the noun 'length' plus the suffix '-en' (a common Old English verb-forming suffix). The word has remained essentially unchanged for over a thousand years.
The suffix '-en' is a hidden superpower in English—it can transform almost any adjective into a verb: darken, brighten, weaken, strengthen. Lengthen shows this ancient pattern still working perfectly, turning a quality (length) into an action (lengthen).
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