The law tries to make common sense of life. An advertiser who claims extravagantly that they have the best product ever will never be held to account by some scientifically minded and disgruntled person, since the claim is what lawyers quaintly call "a mere puff". No reasonable person will take the claim literally. But when the famous Mrs Carlill sued the Carbolic Smoke Ball Company , she won because the company had said that they would give a cash prize to anyone who used their product and caught influenza or a cold, such was its efficacy. They had strayed beyond mere puffery and into the realms of contract.
So it is with bad language. A claim that a man is illegitimate or a masturbator, or both, if expressed in the vernacular of the streets, is not to be taken literally as a claim of truth; it is mere vulgar abuse and the courts, and the world, do not take it as anything more. But, in days when illegitimacy was considered a stigma, saying in terms that a man was illegitimate would have been actionable because the word used, 'illegitimate' was meant, by its very formality, to be taken as intended and true.