feeling of helpless isolation; desolation of a fallen person
Literary four-character compound evoking the image of a solitary castle under a setting moon; used for the loneliness and helplessness of someone who has lost status, fortune, or companions.
After going bankrupt, he felt exactly like a solitary castle under a setting moon—utterly alone and helpless.
With his supporters gone, the aging politician was overcome by a deep sense of desolation.
孤立無援 means 'isolated and without help' in a more concrete, situational sense, while 孤城落月 is a poetic metaphor for the emotional state of a ruined or abandoned person.
落日 literally means 'setting sun' and can metaphorically refer to decline, but lacks the specific nuance of lonely helplessness carried by 孤城落月.
A yojijukugo (four-character idiom) combining 孤城 (solitary castle) and 落月 (setting moon). The exact origin is uncertain, but it likely derives from classical Chinese poetry, where such imagery symbolizes isolation and the passing of glory.