He could just as easily have said "Arseholes" or "shitheads". It's vulgar, but a common vulgarity... like saying "Oh bugger it!" (Buggery being the act of anal penetration for the sexual gratification of at least one person and that term being a common expletive used to comic effect by the character Death in Terry Pratchets' Disc Word series of novels) The frequent use of the word does dull the shock factor of it's vulgar nature, but we still use it in it's correct sense, as well as an insult or curse, so it doesn't become less vulgar.
It isn't actually any less vulgar for me to say "poo!" (as I often do) than to use the famous four letter "S" word. It just doesn't have those sharp "sh" and hard "t" sounds to it, so when thrown as a curse it has a comic effect rather than a scathing one. It still refers to the same messy, smelly, brown stuff though.
Yes, "illegitimate" is a correct equivalent. In contemporary use, it is more commonly applied to "wild" children, (those conceived in communes of "free love" where monogamous relations are shunned and children are considered a "communal" responsibility) those born to absentee fathers, (those who are not seen or heard from after the conception is announced) or the offspring of "loose" women who are unable to keep track of the partners who may have been responsible for their confinement. (or, at least 50% responsible XD)
With such non-slang uses, you can understand
why it is used to "put down" a person you do not like, and why it is, and should always be, quite vulgar. (unless we all convert to one big "free love" commune... as that is the only society where a man failing to accept the responsibilities of parenthood should be acceptable) In a modern society, where masturbation is no longer seen as a symptom of mental disease, devil worship or even sexual ineptitude, calling someone a "wanker" is actually far less vulgar. You may even be able to find a stand up show where
Jasper Carrot famously mimed that hand movement and lip read the word as see through a car window when someone doesn't like his driving style, and then switches back to himself to say "Who? Me?... Yes I am thank you." with a big grin. :D:
