"mercall" means: A grain measure in use in the Madras Presidency, varying in different localities, but typically 12 sers; fixed since 1846 at 800 cubic inche.... 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.