Etymology
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underscore (v.)

1771, "to draw a line under," from under + score (v.). The figurative sense of "to emphasize" is attested from 1891. Noun meaning "a line drawn below (something)" is recorded from 1901.

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Definitions of underscore
1
underscore (v.)
give extra weight to (a communication);
underscore (v.)
draw a line or lines underneath to call attention to;
Synonyms: underline
2
underscore (n.)
a line drawn underneath (especially under written matter);
Synonyms: underline
From wordnet.princeton.edu