Green’s Dictionary of Slang

1. rain.

1924		Mail (Adelaide) 16 Feb. 1/4: If it begins to rain [it is] the King of Spain.
1955		Argus (Melbourne 5 Feb. 3/2: No chance of any King of Spain over the Khyber Pass.
1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxv 6/3: king of spain: [...] rain.
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 33: King of Spain Rain.
2002		Pete’s Aussie Sl. Home Page 🌐 King of Spain: rain.

2. a train.

1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxv 6/3: king of spain: Train.
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 33: King of Spain [...] train.

3. an airplane.

1967

‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxv 6/3: king of spain: [...] plane.