Unknown. Possibly a calque of Japanese 胸襟を開く (“to share one's feelings”, literally “to open one's collar”) or Japanese 裃を脱ぐ (“to be frank”, literally “to take off one's haori and hakama (two garments worn over a kimono)”).
释义
动词verb
1(idiomatic, often business) To revealconfidential information, especially details of one's business operations.