our old friend Michael Quinion at World Wide Words provides this A:

A. I am indebted to the American Dialect Society Web archive, and in particular to
Jim Rader, for the answer to this question, which otherwise I couldn't find in any of
my reference books. The word Dopp is a registered trade mark of a man's toiletry
kit. It was designed by Jerome Harris for his uncle Charles Doppelt, a German
immigrant to Chicago in the early 1900s. So it's presumably an abbreviated form of
Mr Doppelt's family name. The word became widely known during the Second
World War when GIs were issued Dopp kits. The company was purchased by
Samsonite in the early seventies.