Gearbox is a term which is mentioned after someone has been shut down seriously or snapped pretty well by a quick or witty comeback. Although there doesn't seem to be a connection, this term is derived from the snapping sound a gearbox transmission gives when the gears aren't engaged properly.
"Hey mate, didn't you play soccer with my brother the other day?"
"Nope. But I played soccer with your mum the other week. Oh snap!"
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