Landsharking

The practice of approaching one's ladyfriend at speed, leaning forward with hands together on one's head as a shark fin, ending with the insertion of this fin into her front bottom.

Jane: Hey Amy, how was the film last night?

Amy: Not bad. I guessed the ending. Tell you what though, Rob gave me a good old landsharking when I got home.

Jane: What a toerag!