"fireman's flap" means: The buttoned or snapped flap at the rear of a union suit — the old-school one-piece long underwear favored by laborers and working men. Call.... This is a fairly neutral word with no inherent risk attached. There is no real cause for parental concern; it is descriptive vocabulary rather than something dangerous. If your child uses it, context will usually make the meaning clear. A brief, curious question about where they heard it is generally enough to know whether to follow up.