Seaman's Slops. Museum Of London.

According to the museum of London this style of clothing was worn by seamen from the 16th century (1500s) into the 18th century (1700s).

Front

Back.

Notice the pattern on the main body of the frock. This is apparently made from Lindsey Wolsey, a mix of linen & wool.

http://collections.museumoflondon.org.uk/Online/object.aspx?objectID=object-83031&rows=1&start=2&sort=summaryTitle%20asc;