下線
かせん · kasen · JLPT N2 · noun
underscore
✏️ 下 (under) + 線 (line) = a line placed under text
下 means 'below' and 線 means 'line', so 下線 is literally a line drawn beneath words. Read it as 'under-line' and it sticks.
下 — horizontal line 一 + mark below
下 is an indicative character: a long horizontal line represents a reference level, and a shorter stroke placed beneath it points to the area below. The original meaning 'below, down' comes directly from this spatial diagram, and it later extended to verbs like 'descend' and 'lower'.