Green’s Dictionary of Slang

1. a nickname for a stupid person.

1902		Sun (NY) 12 Oct. 18/2: Dippy lost out on his job .
1935		S. Kingsley Dead End Act I: H’lo Angel! H’lo Dippy!

2. (W.I.) a deportee from the UK.

1995

Francis-Jackson Official Dancehall Dict. 14: Dippy a deportee: u. a British dippy/a deportee from Britain.