expression, noun
close adviser; confidential retainer
Historical term for a trusted military or political adviser who served a lord from within the headquarters tent. Rare in modern Japanese; encountered mainly in historical texts or period dramas.
帷幄の臣として、彼は将軍に作戦を進言した。
As a close adviser, he proposed a strategy to the general.
From 帷幄 (いあく, 'field curtain; headquarters tent') + の (possessive particle) + 臣 (しん, 'retainer; vassal'). The variant 帷幄之臣 uses the classical possessive particle 之. The exact historical derivation is uncertain, but the term is associated with ancient Chinese and Japanese military traditions.