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 forfeit 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.
References
Piru (spirit) Wikipedia(Text) CC BY-SA