Green’s Dictionary of Slang

gumby n.2

[the green character, Gumby, a toy, portrayed by Eddie Murphy on U.S. TV’s Saturday Night Live and/or the character in BBC TV’s Monty Python’s Flying Circus (1969–74); echoic of his monosyllabic incoherence]

a fool, an idiot.

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Head of the Class [ABC-TV] I’m still stuck in here with these gumbies [HDAS].
[US]Eble Campus Sl. Mar.
[Aus]T. Winton Lockie Leonard: Scumbuster (1995) 79: ‘Well,’ said Lockie. ‘Paint me green and call me Gumby. [...] I thought you were fourteen.’.

In compounds

gumbyhead (n.) [-head sfx (1)]

(US campus) someone who does something stupid.

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[US]Eble Campus Sl. Mar.
Jonetta ‘Maxwell’s Story’ on Whiskey Hollar 🌐 What am I nervous about, anyway? Afraid of looking like a total Gumbyhead?