expression, noun
dissolute mind; hardened heart
Describes a heart or mind that has become coarse, callous, or morally degraded, often through hardship or a dissolute lifestyle. The phrase carries a literary or slightly formal tone.
長年の孤独が彼の荒んだ心をさらに冷たくした。
Years of loneliness made his hardened heart even colder.
荒んだ心を癒すには、自然に触れるのが一番だ。
To heal a dissolute mind, the best thing is to be in touch with nature.
The verb 荒む means 'to become wild/coarse/desolate'; 荒んだ is its past attributive form, so 荒んだ心 is literally 'a heart that has become desolate'.
A more common verbal phrase meaning 'one's heart becomes desolate'; 荒んだ心 is the resulting state expressed as a noun phrase.
From the past attributive form of the verb 荒む (to become wild/desolate) + 心 (heart/mind). The expression is a straightforward combination of these elements.