Dagenham? You come from ... where?

"Dagenham? You can't possibly come from Dagenham!" a flamboyant acquaintance in the West End of London once told me. "You simply must tell people you come from 'Darn-em' and you must place your hand over your mouth as you say it, just in case."
Just in case? In case of what? I felt an urge to defend the place, but then thought better of it.
Dagenham. "Call it Daggers," said another wag. Well, I called it home.