bastardo

The historical sense is the one that earns the page. In law and in history writing a bastardo is a child born outside marriage, and for most of European history that was a legal status with consequences attached: no inheritance, no title, a separate line in the register. That is what the word is doing in every text you will meet it in, and it is a fact about property law rather than an opinion about the child. The adjective is bastarda and bastardeco is the condition.

The most famous holder of the title. William, Duke of Normandy, was Vilhelmo la Bastardo long before he was Vilhelmo la Konkeranto, and the by-name was used to his face by people who wanted to annoy him. Heraldry had its own way of saying the same thing without saying it: the bend sinister, a band drawn across the ŝildo from the top left corner down to the bottom right, marking a line of descent as illegitimate while keeping it inside the family arms. The bar sinister of English novels is a misremembering of that device.

It is also a biology word, and that sense is entirely neutral. A bastardo is a hybrid: a cross between two species or two breeds, plant or animal. Bastardigi is to crossbreed. Nothing pejorative attaches to that use at all, and it is the sense you will meet in a gardening book.

On the insult. English bastard is now used far more often as abuse than as a description, and the Esperanto word can be used that way too. That is worth knowing and not worth practising: the historical and biological senses are what the word does its work in, and this page will leave the swearing to somebody else.