The song was achingly sad, bringing tears to every listener's eyes.

achingly 是由动词 ache（疼痛）加上副词后缀 -ingly 构成的副词。ache 本意是"疼痛"，引申为"渴望、痛苦"。achingly 表示"令人痛苦地、极度地"，用来强调某种情感或状态的强烈程度。