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.
It originated from the term "I'll be back in a flash", but with the word "flash" replaced with "flask". It gives a double meaning now, where you can say "I'll be back soon" and also "I'll be back inside an alcohol containing vessel".
"Hey, dude, I'm gonna go pick up some more ammunition at K-mart. I'll be back in a flask."