Green’s Dictionary of Slang

1. (Aus./US) a fool; thus melon-headed, stupid.

1914		Bulletin (Sydney) 2 July 24/4: Do ut, thin, [...] ’n’ do ut quick if you’re thinkin’, or that shlit-mouthed, melon-headed Orange oaf, conshtable McCracken in thinkin’, a bit iv a warrant is a shootable coshtume fer a lady t’ be paradin’ the streets in.
1938		West. Mail (Perth) 6 Oct. 48/2: I still believed I had a chance, only my melonheaded mate blurted out something about the way I disappeared.
1947		N. Algren ‘El Presidente de Méjico’ in Texas Stories (1995) 86: There’s always the right way to do a thing, Melonhead.
1999		K. Sampson Powder 477: James looking like a melonhead in the foreground, dressed in formal evening-wear and cummerbunds.
2001		(con. 1975–6) E. Little Steel Toes 10: You gonna scuff those shiny boots all up kicking those sons of bitches in their melon heads.
2003		Guardian 8 Sept. 🌐 The officious supervisor bloke quickly halted his progress and confiscated the flag, to a huge cheer from the butchers, pirates and melonheads in the crowd.
2007		M. Rowson Stuff 45: The front row is filled with cabbage-faced, melon-headed party stooges.

2. a bald person, thus melon-headed, bald.

1924		Gippsland Times (Vic.) 15 Sept. 4/4: Bald-Headed Men Wanted! [...] A caller of the melon-headed variety.
1937		West. Mail (Perth) 11 Nov. 40/3: My poor old melon-headed mate tried it.
1942		Cairns Post (Qld) 28 May 6/3: I heard a yell of ‘Wake up, melon-head!’ from a youth aged 10.