Dagenham, adj. — Green’s Dictionary of Slang

Dagenham adj.

[London Borough of Barking and Dagenham in East London, thus pun on barking adj.]

mad, eccentric.

1999		Guardian Christmas 99 24 Dec. 4: ‘That man Filbone was absolutely DAGENHAM.’ ‘Yes...two stops on from BARKING!!’.
2020		Twitter 9 Apr. 🌐 That's him full fledged 100% wacko nut job, way past Barking in fact Upminster.