noun

• (informal, usually uncountable) A thick, slimy substance; goo.
  "The flesh [of the mistletoe berry] is sticky, and forms strings and ribbons between my thumb and forefinger. For the mistletoe, this viscous goop – and by the way, viscous comes to English from viscum – is crucial. The stickiness means that, after eating the berries, birds often regurgitate the seeds and then wipe their bills on twigs – leading to the seeds' getting glued to the tree, where they can germinate and begin the cycle anew."
• (countable, informal, derogatory, dated) A silly, stupid, or boorish person.
  "I pointed out that Stiffy, who is pure padded cell from the foundation up, was planning to marry the Rev. H.P. Pinker, himself as pronounced a goop as ever preached the Hivites and Hittites"

verb

• (informal) To apply a thick, slimy, or goo-like substance.
  "Airstream, 1977. He showed her ones on eBay that looked like blunt rifle slugs, went for crazy money in any condition at all. The uncle had gooped this one over with white expansion foam, gone gray and dirty now, to stop it leaking and for insulation."
• (informal, dated) To stare; gawk.
  "Young John blushed at being found “gooping” at himself in the glass."