>Title 18 of the United States Code, Section 1464, prohibits the utterance of any obscene, indecent or profane language by means of radio communication. Consistent with a subsequent statute and court case, the Commission's rules prohibit the broadcast of indecent material during the period of 6 a.m. and 10 p.m. FCC decisions also prohibit the broadcast of profane material between 6 a.m. and 10 p.m. Civil enforcement of these requirements rests with the FCC, and is an important part of the FCC's overall responsibilities. At the same time, the FCC must be mindful of the First Amendment to the United States Constitution and Section 326 of the Communications Act, which prohibit the FCC from censoring program material, or interfering with broadcasters' free speech rights.
>Obscene speech is not protected by the First Amendment and broadcasters are prohibited, by statute and regulation, from airing obscene programming at any time.
>The safe harbor refers to the time period between 10 p.m. and 6 a.m., local time. During this time period, a station may air indecent and/or profane material. In contrast, there is no safe harbor for the broadcast of obscene material. Obscene material is entitled to no First Amendment protection, and may not be broadcast at any time.
To your point:
>Are there certain words that are always unlawful? No.
https://www.fcc.gov/reports-research/guides/obscenity-indece...