A neutron bomb.
The synthetic hallucinogen 25I-NBOMe, used as a recreational drug.
A negation word, such as not, nobody, or nothing.
The word nigger or nigga or nigra.
A Danish/Swedish/Norwegian noun of the common gender (their indefinite article being en), as opposed to t-words, nouns of the neuter gender (their indefinite article being et).
Any word beginning with n that is not normally taboo but is considered (often humorously) to be so in the given context.
The word Nazi.
Any word, regardless of its starting letter, that is used oppressively, marginalizes or oppresses a group of people.