Green’s Dictionary of Slang

dilly adj.²

delightful.

1909		Punch 26 May 362/1: I sent out the ordinary cards [...] with ‘Dancing’ in one corner of the card, but in the other corner was ‘Bare feet’. Wasn’t it a dilly idea?
1922		C.E.M. Joad Highbrows 103: Have you heard that new waltz, ‘Luscious Love’? It’s simply dilly [OED].
1984		D. Jenkins Life Its Ownself (1985) 284: ‘When will this be on TV?’ Priscilla asked [...] ‘Tomorrow before the game,’ I said [...] ‘Dilly!’ Priscilla said.