Piru (spirit)

A piru is a fiend or demon in Finnish mythology. In folklore, a piru is often featured as a nasty spirit of the forest with which a wise Alec either wins or loses a battle of wits, giving or receiving a in return. In many cases, poltergeist and haunting phenomenons are described as "pirus". The Devil may be referred to as (proper noun) Piru, or Pääpiru, the main piru.

"Piru" is also a mild swearword in Finnish.

...
Wikipedia