The act in which an individual sneaks into their brothers room, well past midnight, to profusely engage in unholy homosexual actions. However, to completely reach the "Voyack" state of "voyacking", one's mother must walk in on the action and join in on the festivities. Anyone caught "voyacking" in the mid 1750s would be burned at the stake without trial.
Did you hear about that family? They were caught voyacking and were publicly disembowled as punishment.