'"Navvie" is English slang for unskilled labourer. 
The original navvies were the (often Irish) labourers who dug the
canals. A "navigation" was a river turned into a canal by widening,
straight links instead of meandering etc, 
When the railways were built the construction labourers were also called
navvies. 
Eventually it became used for any unskilled heavy manual labour.  '

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