"on yer bike" means: A British idiom meaning 'get lost', 'scram', or 'you're not welcome here' — said with varying degrees of heat depending on the situation. It.... 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.