viedä saunan taakse

There are competing theories:

• Based on the tradition of animals being slaughtered behind a sauna, so as to be further away from the rest of the family to avoid shock or commotion. Saunas would usually be located further away from the main house building to prevent a possible fire from spreading.
• A potential threat to children that unless they mend their ways, they would be taken behind the sauna to be spanked or whipped as a form of punishment, also done behind the sauna to prevent cries from being audible back home.
• Another theory suggests an origin dating back to the Finnish Civil War, referring to extrajudicial executions possibly taking place behind a sauna.

The exact meaning depends on the speaker, and it's more than likely that there being multiple theories about the origin has contributed to there being different meanings.

viedä saunan taakse (literally, “to take behind a sauna”) (colloquial)

1. (idiomatic, transitive) to kill, put down, execute (usually implied by shooting); take someone out back
   • 1981, Pauli "Juice" Leskinen, “Ilomantsi”, in Ilomantsi / Syys, performed by Juice Leskinen Slam:

     Ei voi virkavaltaa muuten opettaa
     kuin viedä saunan taakse ja lopettaa

     The only way to teach the authorities a lesson
     is by taking them out back and putting them down

2. (idiomatic, transitive) to beat up, usually as a form of punishment, but not necessarily to death

Occasionally used as a part of a phrase where the action is also explicitly stated, such as viedä saunan taakse ja tappaa (“to take behind a sauna and kill”).

See viedä.