Imperial General Headquarters announcement; official announcement (often with a nuance of propaganda or unreliable information)
Originally referred to official announcements made by the Imperial General Headquarters during World War II, which were often propagandistic or misleading. In modern usage, it can refer to any official announcement that is perceived as unreliable or self-serving.
During the war, Imperial General Headquarters announcements often differed from the facts.
政府の説明はまるで大本営発表だ。
The government's explanation is just like a propaganda announcement.
プロパガンダ is a broader term for propaganda in general, while 大本営発表 specifically evokes the historical Japanese wartime announcements and their perceived unreliability.
Compound of 大本営 (Imperial General Headquarters) and 発表 (announcement). The term gained its modern connotation from the perception that wartime announcements were often distorted or false.