To draw a line beneath text to emphasize it, or figuratively, to stress the importance of something.
Compound of "under" + "line", first used in the printing sense in the 1700s. The figurative meaning "to emphasize" developed by the 1890s as the visual practice became associated with highlighting importance.
Before digital bold and italic formatting, underlining was the primary way to add emphasis to handwritten and typed text, making it one of the most democratically accessible forms of visual emphasis. The metaphorical extension of "underlining" to mean emphasizing anything shows how physical writing practices shape our conceptual language about importance and attention.
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