Nissen hut means: A prefabricated building, formerly used by the military, having a semicircular roof of corrugated iron.. It is military jargon and mostly appears in service contexts, war fiction, or among veterans and their families. There is no real cause for parental concern; it is descriptive vocabulary rather than risky behaviour. If your teen uses it, context will usually make the intent clear. A short, curious question about where they heard it is usually all that is needed to know whether to follow up.