noun, from Finnish, meaning "limp", a 'lerppu' is the name for a floppy disk, but it is usually associated with penises, so a lerppu or lerppu kyrpa would be someone who has a limp penis. 

"Dude, you threw away my homework?  You lerppu!" 
It can be used to talk about penises, like after a long date, the man and woman finally get somewhere, and the man is devoid of an erection; he has a lerppu.