When you're wiping after a shit, and even after what feels like hundreds of wipes, there is still brown on the toilet paper. It is as if you are wiping ink off of a brown marker.
Person 1: Bro, what took you so long? That was like a 40-minute shit!
Person 2: Sorry bro, I had ink ass.
When you stand up on the shitter before you wipe. The shit then is pressed between your ass cheeks thus giving the ink blot effect. When you wipe, it makes the same pattern on both sides of the toilet paper.
Dude, check out the pattern on the toilet paper from the ink blot effect I got when you knocked on the door!
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.
"I loved the diamond engagement ring he gave me. When I saw it, it made me ink the seat"