(Flintshire). A corruption of Wato's Dyke. Wato was the father of Weland, the Vulcan of Northern mythology, and the son of King Vilkinr by a mermaid. This dyke extends from the vicinity of Basingwerk Abbey, in a south-easterly direction, into Denbighshire. The space between it and Offa's Dyke, which in some parts is three miles, and in others not above 500 yards, is neutral ground, “where Britons, Danes, and Saxons met for commercial purposes.” (See Wans Dyke.)
There is a famous thing Called Offa's Dyke, that reacheth far in length. All kinds of ware the Danes might thither bring; It was free ground, and called the Briton's Strength. Wat's Dyke, likewise, about the same was set, Between which two both Danes and Britons met, And traffic still.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894