Etymology
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underline (v.)

1721, "to mark underneath or below with a line," from under + line (v.). Similar formation in Dutch onderlijnen. Related: Underlined; underlining. The noun is attested from 1888.

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