Bum bag means: A small pouch attached to a belt that is worn on the outside of clothing around the waist and used for holding small personal items.. It is mainly Irish English and may not be understood outside that context. 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.