Visual: Two dull, staring eyes, perhaps: oo. What’s the g? A nose? Off to the side? Naw. Maybe if it were ogon. But then the n? An ear? Who knows. The oo is the dominant feature; it shows up also in kook and loon and goof and tool but also in moon and book and look.
In the mouth: This is a hollow, echoing back-of-the-mouth word that closes off at the tip of the tongue. It has no crispness to it – no /k/ as in cool and coon; just the dull voiced stop /g/, and the resonant /n/ at the end making it sound much like a beat on a big drum.
Echoes: The above-mentioned oo words, such as goof and loon and kook and noodle perhaps, oh, and goose. Maybe go on. There’s no significant taste of good that I can find, but perhaps of gun. And lagoon, though that’s a very different image.
Etymology: It may be a shortened form of goney ‘booby, simpleton; albatross’, which has become gooney bird. Goney may be related to gawney ‘simpleton’; that seems connected with gane, noun, ‘ugly face’; that is likely related to gane, verb, ‘gape, yawn’, which is related to yawn. That’s a bit of a goose chase! But what we know for sure is that our modern use of it traces to Alice the Goon, a character in the comic strip Thimble Theatre, drawn by E.C. Segar and later named after its chiefest character: Popeye. Alice the Goon was a giant, not-exactly-human, thug-like being. You can see a couple of cartoons with her at A View from a Goon.
Collocations: It shows up in the comedy show The Goon Show (featuring Spike Milligan, Peter Sellers, and Harry Secombe; the name was inspired by Alice the Goon). The most common collocation in normal usage is goon squad. As Edward Banatt (@ArmaVirumque) has reminded me, there’s a line in “Fashion” by David Bowie that goes “We are the goon squad and we’re coming to town. Beep beep!” There’s also a novel by Jennifer Egan titled A Visit from the Goon Squad, which won the 2011 Pulitzer Prize.
Overtones: The word is unavoidably insulting (unless the person described knows it’s a reference to The Goon Show, and maybe even then) and carries something of an implication of low intelligence and perhaps subhuman nature. Goons are often seen as subservient lackeys, as in “Call off your goons.” And generally they’re huge and muscular.
Semantics: A goon is a stupid individual, probably violent, probably subservient. Thug is sometimes synonymous. For the rest, see above.
Where to find it: I suspect you’ll find it most often in crime fiction, but that’s just a guess on the basis of the types of people who are in such works – not just the goons but the people who are calling them goons as opposed to anything else. You wouldn’t find goon in a news article except in a quote, but it is the sort of word likely to be quoted. In other words, it’s provocative.