Rude, sexist and possibly racist jargon, that reflects a certain ignorance of Japanese culture; not wise to use. The origin of the term is the idea that Japanese Geisha wear very classic and complex Japanese kimono. The suggestion is that one does not really know what a geisha really looks like until “she opens her kimono.”
The phrase is used in financial and corporate transactions to denote the idea that key aspects of a deal may be kept confidential by each party until the transaction has progressed significantly, whereupon the “kimono is opened” and the key information disclosed.