noun. Like a poodle-in-law, but with slightly less pooping on the sidewalk.
Real citation: “I actually suggested to my mom several days ago that we buy that book for my sis, but I can see that the BIL needs to read it first. If I have to, I will not only beat him with the book, I will have my brothers video-tape it and post it on YouTUBE! (just kidding, but it could happen, if butthead-in-law doesn't cooperate).”
(Nov. 13, 2007, Conservative Underground, http://www.conservativeunderground.com/forum/showthread.php?t=110709)
Made-up citation: “According to the critical-period hypothesis, if I don’t smother my butthead-in-law with a pillow this weekend, the baby Jesus will cry.”