After hearing the same lame excuse over and over again about why the university student couldn't bring their books back on time and why they shouldn't have to pay overdue fines, the librarian wrote "L.L.P.F." in the student's library account so that the other staff wouldn't excuse any more overdue fines.
It's a reference to a gif on the internet with two pitbulls fucking until one pukes and starts to eat it, saying that you are indeed interested in getting down and fucking like puking pitbulls.