the.com/underline

means To draw a line beneath a word or phrase to mark it as important, or more broadly, to emphasize or confirm a point.

from A straightforward compound of 'under' and 'line,' first used literally for the mark drawn beneath writing to flag it for attention. The figurative sense — to underline a point in conversation or argument, with no pen involved — grew naturally out of the physical gesture, spreading as the act of underscoring became shorthand for emphasis itself.