Pantless means: The straightforward informal adjective meaning without trousers or underpants. Used in humorous or literal contexts. Not particularly slangy but informal in register; typically appears in comedic. It is informal slang, fine in casual speech but not suited to formal contexts. 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.