v)
During vaginal intercourse, most usually from the rear, when the penis slips out of the vagina and upon attempted reinsertion, instead pokes the cheek of the buttocks.
to have sexual intercourse not considered offensive enough to be censored from network TV
The OED dates boink as a verb back to 1984, citing Stephen King's "Thinner," where it appears to be used as onomatopoeia, similar to bonk: "He half-expected them to begin bopping and boinking each other."
This precedes usage in the 1985 episode of "Moonlighting" titled "The Next Murder You Hear."
David: It makes you crazy to think that anybody out there is boinking when you can’t.
Maddie: I can't hear you.