Penetrative anal sexual intercourse. Usually performed by homosexual men as intercourse or by heterosexual couple, and it is performed for pleasure rather than reproductive purposes (unlike vaginal intercourse).
Peter: "Hello Marty, do you want to have Butt secks today?"
AS INTERNET DISCUSSION FORUM SLANG:
Gary: "MAZZINA95 TOTLY LKes BUTTSECKS!!!!"
Tim: "Get away you spammer"
AS CONVERSATION DURING FAMILY DINNER:
Father: "So Jen, have you decided what university you want to go to?"
Jen: "yes father, the UNIVERSITY of BUTTSECKS!! I HATE YOU, and I'm NOT going to study to become a lawyer as you want me to!!
Father: "So you want to have butt secks as your profession.. well, a lot of whores do make a lot of money, so I do not see why not"