Green’s Dictionary of Slang

1. (US) something or someone considered the very best; also ironically.

1937		N.Y. Age 25 Sept. 9/1: No-one ever talks of patriotism now [...] It’s racism, [...] Nazi-ism and even Africanism (That last one is a killer-diller, Mater. Pardon the slang).
1941		‘Digg Mee’ ‘Observation Post’ in N.Y. Age 15 Mar. 9/6: ‘[H]is’ Miss Miller is a solid killer-diller.
1944		Cab Calloway New Hepsters Dict. in Calloway (1976) 257: killer-diller (n.): a great thrill.
1953		W. Fisher Waiters 78: Good evening, Mrs. Faulks. You sure are a killer-diller this evening.
1964		Dly Mirror (London) 27 Apr. 14/2: [headline] A killer-diller of a chiller!
1969		B. Beckham My Main Mother 178: Mitchell, you’re a killer-diller, honey, I swear.
1990		R. Gool Cape Town Coolie 124: He used to be a killer-diller, that guy.

2. (US) the individual in charge.

1955

B. Appel Tough Guy [ebook] Charley Valinchi, his God damn boss! Charley Valinchi, the God damn bastard of a killer-diller!