The word 'underline' is a transparent compound formed from two native English elements. The prefix 'under-' derives from Proto-Germanic under, which has a stable history of denoting a position beneath something. The root 'line' originates from Latin līnea ("linen thread, string"), which entered Old English via Proto-Germanic. The compound's literal meaning, "to draw a line beneath," directly reflects the sum of its parts. Over time, this concrete action evolved into the metaphorical sense of giving emphasis or stress to an idea, mirroring the visual prominence created by the line on the page.

Structure: under (beneath) + line (stroke, mark) - under: From Old English under, meaning "beneath, below." It functions as a prefix indicating position or degree. - line: From Old English līne, meaning "cord, rope; series, row." It functions as the core noun/verb element, here representing a stroke or mark.