No, it's no slang. Entrails are, in plural, "inälvor". You can see the "in" there and "älv" is a Swedish word for river.

edit: I do not know the etymology of the Swedish word for entrails, but I'd wager it's just coincidence that it is seemingly made up of two other words. It's not unlike the English word "searing" seemingly being composed of "sea" and "ring".