Earlier I took a look at the rise of slang words that assume nobody trusts you -- "honestly," "literally," "seriously," "I'm not gonna lie," etc. They're all from the '90s onward, part of the broader social trend toward lower trust in people you meet in everyday life. If you expect your listeners not to trust you, you have to continually reassure them that you're telling the truth.
Well, there's a flip-side to that as well: the rise of slang words that signal how little trust you have in others. This reminds your conversation partners that, in general, you're a very suspicious person, so if they really do mean what they say, they'd better mark that overtly with one of the many "honestly" kind of declarations.
"Sketchy" is the first that I remember, and that was from the '90s. I think before then it used to mean "hazy," as in "the details of the robbery are sketchy for now, but police are interviewing witnesses to fill in the blanks." I'm pretty sure that in the '90s it was used primarily for places, particularly unfamiliar public places, that you found suspicious. "I dunno, this place looks kinda sketchy," or "Holy shit this neighborhood is sketchy, let's get the fuck outta here."
Only in the 2000s did it get used to refer to people, as individuals or groups, who are unfamiliar. "That guy who keeps checking you out looks kinda sketchy," or "I thought about rushing Omega Mu, but those girls seem pretty sketchy." It's typically girls who use these low-trust words, as females are less trusting and fearful of strangers and public places than males are.
Then there was "shady" in the '90s, which again I think first referred to a public place or a situation that was not part of your daily routine. "Dude, that McDonalds that we ate at last night in Anacostia was so shady," or "They're having a party Friday night in the woods behind the school? I dunno, sounds shady if you ask me." (Did not mean for that to be a pun.) Now it's more of a term for strangers, as in "That guy who just posted on your Facebook looks pretty shady, to be quite honest."
Also from the '90s and early 2000s was "shiesty" (based on "shyster," with a long-I vowel), meaning someone who presents a trustworthy appearance but is actually devious. It didn't refer to places or situations. I think this is the only one that guys used more than girls did, and it generally referred to other guys -- like ones who wouldn't get your back when you thought they would. "Chris said he forgot his wallet, so we covered his part of the check. Then we see him hailing a cab to get home -- I told you that dude was shiesty."
Two of the most commonly used slang words of the 21st century also refer only to suspicious people or social situations, not places -- "stalker" and "creepy," and their variants (stalking, creeper, etc.). "Stalker" took off in the early 2000s, and "creepy" more like the mid-late 2000s. They primarily refer to male behavior toward females in the domain of dating and mating, whether some guy who makes a pass at a girl, looks like he's going to, or even a guy who shows no awareness of the girl, but if he did, she'd be creeped out. "Who's that fat ugly creeper asleep in the corner over there?"
It's not just a message about how undesirable the guy is -- there were and still are other words for that. Like, "Who's that goofy-looking dork who keeps asking you to dance?" or "Can you believe that loser actually thought I'd go out on a date with him over Brad?" The new words add a layer of connotation that he's not only undesirable but suspicious, threatening, and so on. "OK, not gonna lie, but that stalker sitting at the table behind you, literally looks like he's thinking of raping you." "Ewww, creepy guy number 4 just looked at us again -- I honestly need to go take a shower now."
Their targets are not actually threatening -- probably some awkward, video-game-addicted dork -- but they exaggerate the sense of suspicion and threat in order to fence themselves off from everyone else, no matter how harmless.
Therefore, the rise of "stalker" and "creepy" can be seen as an extension of the date-rape hysteria that began in the late '80s and lasted well into the '90s. But now that boys and girls don't date each other that much anymore, the panic has shifted from the date-rape scenario to creepy guys thinking about or looking at you in the first place, before even approaching you.
In the same way, most of these low-trust words first referred to unfamiliar places, and only later came to refer to people too. The first phase of cocooning behavior was to abandon public spaces, while still having some fellow feeling for your peers (early-mid-'90s). Now with so few people willing to hang out in public, it's common to hear people say that someone is "stalking" them on Facebook, not even in real life. Once you were spatially secure in your narrow private sphere, you would proceed with the second phase by cutting off your social ties to would-be friends and mates.
Cocooning begins once the crime rate rises to such a high point, and most people sense that it's those unguarded public spaces that are most dangerous, so they ditch those first. But you could always be harmed by someone you know and have allowed to get close, so you have to follow up by ditching your peers too, just to be safe.
Honorable mentions go to less popular words like "sus" (from "suspect"), usually referring to someone whose heterosexuality is under suspicion. "I dunno, shaving your balls sounds pretty sus." In this fag-friendly age, it's reassuring to know that at least some of the Millennial generation still finds what they do disgusting. Then there's "seems legit," a self-conscious commentary on what is obviously bogus, polluting, etc., not merely suspicious. "Bro, check out this ad. 'Hot MILFs waiting to fuck you tonight in your city' -- seems legit."
Finally, there's "dodgy," a word that pretentious Americans imported from Britain in the 1990s, judging from its appearance in the New York Times. In British usage, it means risky or chancy, and can refer to inanimate isolated places, like "be careful, the drive up that hill gets a bit dodgy." Americans use it with paranoid and suspicious connotations -- "that new character in Downton Abbey seems a bit dodgy."
The books in Google's digitized library (Ngram) show an initial rise and fall of the word "dodgy" in British English during the Victorian era, beginning in the 1860s. That would've been 20 years after Dickens nicknamed one of his dodgy characters the Artful Dodger. I'm not sure in what contexts it was used back then, as opposed to the contemporary usage of "risky" or "chancy" in general, including inanimate things. But based on the Dickens character, I wouldn't be surprised if it had a connotation of untrustworthiness in human beings, or public places. The Victorian era was another period of low trust and social isolation, following in the wake of the opposite Romantic-Gothic period.