役に立つ
やくにたつ · yakunitatsu · JLPT N4 · expression
to be helpful
🔧 役 (role) + に + 立つ (stand) = to stand in a role, i.e. be useful
役 yaku means role or duty, に marks the target, and 立つ tatsu means to stand. Something that stands up to its role does its job, so it is useful. Same 役 as in 役割 (role) and 役者 (actor).
立 — person standing on ground (大 over 一)
Pictograph of a person standing with arms outstretched on the ground line below. The original oracle bone form clearly shows a frontal human figure planted on the earth. The core meaning 'to stand' extended to 'establish, set up, erect.'