kontraŭleĝa

Three pieces and it defines itself. Kontraŭ is against, leĝo is a law, -a makes it an adjective. Against the law. English reaches for illegal or unlawful depending on how formal it is feeling, and Esperanto does not have to feel anything.

The antonym is already in the course, one chapter back.Laŭleĝa at rank 2,452 is laŭ plus leĝo, in accordance with the law, hence lawful. The two words sit either side of the same noun and differ only in the preposition glued to the front, which is as clean a pair as the language produces. Read them together once and you have both permanently. Malleĝa also exists and is heard, built with the plain opposite prefix, but kontraŭleĝa is the stronger and commoner word: mal- says merely not lawful, kontraŭ- says positively set against the law.

The kontraŭ- family is large and behaves itself. Kontraŭdiri is to contradict, literally to speak against. Kontraŭstari is to resist, to stand against. Kontraŭulo is an opponent, kontraŭa is opposite or contrary, kontraŭe means on the contrary. There is also a preposition-plus-noun pattern worth spotting: kontraŭveneno an antidote, kontraŭvola unwilling.

What the word does not cover. Kontraŭleĝa describes an act that breaks a statute. It is not the word for immoral, which is malmorala, nor for unjust, which is maljusta. The third example above turns on exactly that gap, and so does the laŭleĝa page: lawful describes the rule, just describes the outcome, and a great deal of history lives between them.