To stab someone who you love. Used mostly to describe crazed fans of musicians. They love the band so much, they want to 'love knife' them and stab them. Coined by Kyle Gass from the band "Tenacious D" in their late 1990's HBO comedy shorts.
(Kyle and Jack are stopped by Lee, their biggest fan, outside the club they just performed at. After asking for their home address, the two of them walk away from him with ominous music playing)
Jack Black (JB): "This dude is total stalk-a-razzi..."
Kyle Gass (KG): "He's full on love knife material"
JB: "Love knife?"
KG: "Yeah, you know .. when they get close .. a little TOO close .. 'oh, I love you! I love you! I love you!' .. then *STAB**STAB**STAB** .. ha, and you're covered in blood .. but they think they did a good thing!"
JB: "Creepy man!"