钻心 (zuānxīn) is a verb-object compound most commonly used as a fixed idiom to describe intense, piercing pain — physical or emotional. Unlike typical separable verbs, 钻心 is rarely split in practice; it often appears as an attributive adjective with 的 (e.g., 钻心的疼痛 'piercing pain'). The dictionary also lists 'sneak in; infiltrate', but these senses are not typical for this compound and are better associated with the verb 钻 (zuān) alone.