Green’s Dictionary of Slang

1. (US, also gloop) any slimy, sticky viscous matter, esp. hair oil, sticky sweets, cosmetics.

1918		M.B. Cary Jr ‘Mlle from Armentieres’ 🎵 Oh, Mademoiselle from Niedermendig / Gobbled the Goop for fünfzehn pfennig.
1935		‘The Jargon of trade’ AS X:3 235/2: Goup [sic] [...] is not only a name for the liquid (usually chocolate) that is poured over ice-cream at soda-water fountains, but it is a term current in beauty shops also. In the latter it is a sticky gelatinous fluid used to keep the hair moist while the operator works in a finger wave.
1941		G. Legman ‘Lang. of Homosexuality’ Appendix VII in Henry Sex Variants II 1167: Goop [...] refers to the viscid semen.
1946		AS XXI 74: Goop [...] the mixture in an incendiary bomb.
1958		McCulloch Woods Words 71: Goop [...] Any dirty, sticky, smelly, messy stuff.
1978		S. King Stand (1990) 721: It made a milky, viscous gloop.
1991		B. Hamper Rivethead (1992) 159: You could see her cavorting across American television with a canister of goop aimed squarely at her stylish mass of tresses.
1993		T. Winton Lockie Leonard: Scumbuster (1995) 100: Half a kilo of milk-fed goop wrapped in a pink diaper.
2002		D.H. Sterry Chicken (2003) 54: I lie awake with a thick layer of goop covering my mouth and throat.
2013		T. Robinson Hard Bounce [ebook] Enough goop was caked under her eyes to give her a vaguely raccoonish look.
2020		C. Hiaasen Squeeze Me 74: ‘What’s all that goop?’ [...] ‘The intestines, ma’am’.

2. (Aus.) a fool.

1932

‘William Hatfield’ Ginger Murdoch 105: I bin trying to tell you enough! this last half-hour, you big goop.

3. (US) gelignite, nitroglycerine.

1969		D. Pendleton Executioner (1973) 36: ‘How much goop did you use, Boom?’ [...] ‘You said enough to eject the safe. I ejected it.’.
1972		D. Pendleton Boston Blitz (1974) 85: A pound and a half of ‘goop’ — plastics explosives.