As a verb, to underscore something means to emphasize or highlight it. As a noun, an underscore is the low horizontal line character "_" used in writing or typing.
From "under" and "score," originally meaning to draw a line beneath words to mark or emphasize them. The computer character kept the idea of a line underneath text.
Before bold fonts, people literally underscored words with a line to show importance—your word processor’s underline tool is that old habit in digital form. The little "_" in email addresses is a tiny fossil of handwriting practices.
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