For some reason the phrase "talk a lot of stick" is coming into my mind when I think of a person who talks a lot of shit, but I couldn't find anything when I Googled it.
Is there any phrase which doesn't sound vulgar but describes this action?
Quit talking smack yo.
Smack talk is also a slang term used in sports. It refers to inflammatory comments made by a person or team in order to insult, anger, annoy or be boisterous towards your opponents. Although it began as a term used by sports fans and athletes, it has spread to all areas of culture where competition takes place. In the United States, it is synonymous with "trash talk". –Wiki
You could say that they talk a lot of hogwash if you mean talking shit in the sense of being nonsensical
meaningless or insincere talk, writing, etc.; nonsense; bunk.
Dictionary.com
to talk foolishly or excessively
M-W
"he just blathered on and on"
"she began blathering on about spirituality and life after death"
synonyms: prattle, chatter, twitter, prate, gabble, jabber, go on, run on, rattle on/away, yap, jibber-jabber, patter, blether, blither, maunder, ramble, drivel
informal yak, yackety-yak, yabber, yatter, witter, rabbit, chunter, natter, waffle, slabber, mag
archaic twaddle, clack, twattle
How about talking a lot of baloney?
baloney
Slang. foolishness; nonsense
Origin of baloney: 1915-20, Americanism; alteration of bologna, with substitution of -ey for final schwa
Falsehoods, nonsense, or foolishness. Baloney in this sense might have originated from the word "blarney," which means flattering or nonsensical talk; more usually it refers to bologna sausage, which is blended from different meats, therefore implying content of inferior or dubious quality. Primarily heard in US. What a load of baloney. I know he doesn't have that kind of money.
(Farlex Dictionary of Idioms)
Also, talking a lot of bull and bulling
bull
n. Slang. exaggerations; lies; nonsense.
Idioms
shoot the bull, to talk aimlessly: We just sat around shooting the bull.
Origin: 1620-30; < Medieval Latin bulla play, game, jest, perhaps special use of Latin bulla bubble; now generally taken as a euphemistic shortening of bullshit
tv. slang : to fool especially by fast boastful talk
iv. slang : to engage in idle and boastful talk
“Come again?” “Quit bulling me, Cal. What do you know? What are you getting at ?” “You tell me, old buddy. What's up?"
If you are meaning to suggest it more as someone being relentlessly challenging or confrontational without merit, I first picture "raging on about ...". But though the Google results for it seem to return to a reasonable variety and depth of usage, including a Psychology Today column and a DailyMail "article"... I couldn't find anywhere to officially give it definition\merit. Probably still a more niche phrase, I suppose.
Perhaps bombastic or pugnacious, or some of their synonyms get close to such an idea. As much as anything, it's the root words that draw me to those words. I'm probably the only one with visions of a pig stubbornly sticking with it, or a small dog incessantly barking his head of off (regardless of the fact that pugs aren't that small, and don't bark a ton). I also saw quarrelsome suggested as a synonym, which might just encapsulate certain parts of the idea. But it's definitely a challenge to find a concise word. So indeed, a phrase might be better. Something like "relentlessly hyperbolic" or also "barking (on) about". But others can hopefully come up with much better than that!
Or if you want to go the slang direction, perhaps frontin(g)? Though I have no idea if that's got a vulgar background as well, it seems like it could just come the perhaps historical usage of putting up a front, perhaps from military or protective purposes?
This is from a native English speaker (American)... talking trash is the best non-vulgar yet informal way to say "talking shit". The other answers are either not informal or not true to the typical meaning of "talking shit".
Urban Dictionary has some ideas about synonyms for talking shit, and the best one of these is "talking trash". This is not vulgar but has the same connotations as talking shit. See below for why I think some of the other answers are not as closely related. For the record, I don't think Urban Dictionary has "talking shit" quite right. "Insulting Someone" should be the first definition.
Talking shit has a specific connotation of speaking highly negatively about an individual, a group of individuals, or a thing/things. One could talk shit about a brand of car, a freeway, a city, his or her neighbor, etc. For example, "Kansas City is full of idiots," "Ford cars are the ugliest pieces of shit," "Your mom is a fat bitch," and so on. If you said something like "His mother was rude on the phone," that probably isn't talking shit. If you said "His mother is a fat rude skank" then that is talking shit.
The proof of this is that if you reverse "talking shit" you get another useful version, "shit-talking," as in "I heard you said my mom was a bitch you shit-talking shit eater!" One would never say someone was "shit-talking" when they meant saying things they didn't mean. You can also reverse "talking trash" in the same way and you get "trash-talking". It has all the same connotations of highly negative speech.
The best answer so far is "talking smack". But I'd say talking smack isn't as serious as talking shit, so it doesn't quite work. As @Mazura says, this is often used in a sporting or other competitive, somewhat friendlier setting. "Fronting" is talking shit when you can't back it up, like if you said to someone "I'll kick your skinny ass" but you would actually lose to the person in a fight. @JeopardyTempest, "fronting" is not vulgar at all, but if you tell someone they are fronting you better not be fronting ;)
To address the first definition in the Urban Dictionary link above ("Just bull shitting around with words and saying crap you dont really mean"), I'd say this is rarely used and if you wanted to be vulgar you would just say "bull-shitting" for saying something you don't really mean.
euphemism
: a mild or indirect word or expression substituted for one considered to be too harsh or blunt when referring to something unpleasant or embarrassing.