The word 无动于衷 is formed by sequentially combining the characters 无 (meaning "without" or "not"), 动 (meaning "to move" or "stir"), 于 (a preposition meaning "in" or "at"), and 衷 (meaning "inner heart" or "core"). The construction directly negates movement with 无, applies it to the heart via 于 and 衷, resulting in the literal sense of "no movement in the heart", conveying emotional indifference.