The word **叹息** is a classical Chinese compound formed through the method of **semantic complementation**, where two characters with related meanings are combined to create a more nuanced single concept. The first character, **叹 (tàn)**, primarily means "to sigh" or "to lament," while the second, **息 (xī)**, originally means "breath" or "to rest." When paired, **息** here specifically refers to the physical act of exhaling, so the compound literally depicts the action of "emitting a sigh of lament." Thus, the term synthesizes both the emotional state (lamentation) and its physical manifestation (a sigh), resulting in the comprehensive meaning of "to heave a sigh" or "to sigh with regret or sorrow."