The word 'underscore' is a transparent compound formed from two native English elements. The prefix 'under-' derives from Proto-Germanic under, which has maintained a stable meaning of "beneath" from its Proto-Indo-European root ndher-. The second element, 'score,' originates from Old Norse skor, meaning a notch or incision, which itself comes from Proto-Germanic skurō-, related to cutting. By the 18th century, 'score' had developed the specific typographical sense of a line drawn on a surface. The compound 'underscore' literally means "to mark with a line beneath," a mechanical action that naturally evolved into the metaphorical sense of giving emphasis or stress, mirroring the visual prominence created by the underline.

Structure: under (beneath) + score (line) under: From Old English under, meaning "beneath, among, between." It functions as a prefix indicating a position below or secondary status. score: From Old Norse skor, meaning "notch, tally, twenty." In this context, it evolved to mean a line, especially one drawn. It functions as the core noun/verb element.