'Watford' most likely recalls a ford over the River Colne.
The 'Wat-' may come from Old English waet, full of water, as the area was marshy, or from wath, hunting; Londonist also mentions words for wading, and a theory linking the name to Watling Street. The name is first mentioned as Watforda in an Anglo-Saxon charter of 1007. The town stands where an ancient trackway crossed the Colne.