Its a phrase used by girls to express a pleasurable experience, especially one that involves sexual chemistry and the production of various juices. Like a squid inks, a girl inks, except its not ink. The seat part comes in because generally you have to sit down to hear profound news, usually bad, but in this case it might be an engagement ring. A diamond is the quintessential seat-inker.
by Headcircus April 23, 2013