Its a greentext joke on 4chan that made fun of how expensive the Five-SivenN rounds are.
> walking home one night
> Mugger comes out of nowhere
> "Give me your wallet!"
> Unload my FN Five-seveN on him instead
> Realize that I used 20 rounds of 5.7*28mm ammo
MFW it would have been cheaper to just hand over the wallet