Involuntary semi, caused by an unknown mix of vehicle vibration, women and the inevitable smell of diesel fumes leaking in through the back seats on a crappy public transport. Usually happens in a morning on the way to work.
8:30 AM - the bus is packed. It smells of diesel fumes as you sit on the back seat. The bus is old and vibrates like mental. A hot woman gets on the bus and you're suddenly aware of a semi, but you don't know what's caused it. 'Diesel Nob'