Character from Viz comic, and one of the longest running and best known.
Real name Sidney Smutt, Sid the Sexist, along with his equally gittish mates Baz, Bob, and Joe, live in Newcastle and spend their days drinking Newcastle Brown and lying about the amount of crumpet that they're getting. He wears a knockoff red Ralph Lauren polo shirt on which the crest looks distinctly like a penis, black trousers, and has a daft 1970s perm and gold medallion accessories. He is always bullshitting about his sexual conquests to his mates, who seem to believe him, or, most likely, daren't call him out because they're all bullshitting as well. Because in reality, Sid is fat, stupid, and hopeless with women, who lives with his mother. He also seems to have latent feelings of homosexuality at times, which manifest in the form of often violent homophobia, and has indeed had quite a few homosexual encounters in the strips (albeit unwillingly).
Most of the strips in which Sid appears revolve around his increasingly erratic attempts at pulling women. These are doomed to failure and humiliation, which he saves face on by claiming the gal in general was "probably a lesbian." In the absence of pulling women, he'll try and consider increasingly dangerous ways of getting off his rocks, which are doomed to failure and humiliation. There's the obvious ones, such as putting on a pheromone sex spray that females cannot resist and then finding himself pack-raped by stray dogs, or trying to perform autofellatio and ending up covered head to toe in traction (before lying to his mates about a highly improbably accident he's had, all of whom coincidentally have exactly the same injuries), but more often there's ones where the lass is genuinely nice and seems to like him but for his inability to be anything other than a pervert who can't help but bullshit about his non-existent sex life and/or make crass remarks and suchlike. The "sexist" angle of the title comes from his inability to see women as other than, well, walking spunk-buckets who are there for his convenience, rather than actually people in their own right.
The basic premise behind Sid the Sexist is that the people who go on the most about how much shagging they've been doing and in such detail are, in fact, hopeless at same and couldn't pull a rotten tooth out of a dead horse's head.
Thing is, I've seen several real-life Sid the Sexist equivalents in action. Granted, none of them spoke in a form of Geordie so thick that it'd need subtitles (in the comic it is written in Geordie, phonetically, so much so that in one strip the background music in a nightclub went, "howay y'buggaz everybody get doon, Friday neet, wi' gan oot on the toon,") but the intent was there. In pubs and clubs and Vertical Drinking Establishments in the provinces I have seen people attempt such chat up lines as, "how pet, dee ya like jewelry? Well ye can suck mi cock it's a gem!" and, "how pet, how do ye like yer eggs in the morning? Fertilised?" and, depressingly, not get spat at, slapped, or kneed in the knackers. In fact, the structured reality TV show Geordie Shore seems to model itself on Sid the Sexist. And the lads from said programme allegedly score in this manner. And yes, I am well aware that several "stars" of that august program are really mackems or dooramites or sand dancers, so don't bother.
There's also a rather uncanny similarity between Sid the Sexist's disposition and that of the pick up artist community. Both of them not only have unhealthy attitudes towards women, and both of them also rely on increasingly esoteric and zany schemes to somehow compel women to fall into bed with them and then lie about their success at same. And also, both are keenly aware that this sort of thing is a numbers game and that the skill is in picking one's targets (who is of the lowest self esteem in the place), and/or taking a scattergun approach to it - you go to a horrible meat-market of a club and try your daft mind games and stupid lines on everyone systematically and eventually you'll find someone daft or desperate or possessed of low enough self esteem to fall for it. Also, Sid is frankly thick enough to actually sign up to their massively overpriced seminars as well.