Green’s Dictionary of Slang

diddy n.¹

a gypsy.

1959		I. & P. Opie Lore and Lang. of Schoolchildren (1977) 76: Gypsies, usually known as ‘gyppoes’ or ‘hoboes’ (or ‘diddies’ in the west of England, and ‘tinks’ in Scotland).
1984		Partridge DSUE (8th edn) 306/1: late C.19–20.
2000		Guardian 20 Apr. 3: He doesn’t like caravan people – or Gypsies and diddies as he calls them.