The Unabridged Dictionary shows definitions for five different senses of sectarian, but the relevant sense is the one meaning "of, relating to, or having the characteristics of one or more sects especially of a religious character." In the news reports, the religious sects are Islamic, but this use of sectarian was born not in Islam but in Christianity.

The first appearance of the original sectarian dates to the mid-17th century, a century after the establishment of the Church of England. Originally, sectarian meant "of or relating to adherents of a religious sect held to be heretical or schismatic," but the word was frequently used to refer to dissenters from the Church of England and specifically Independents and other Protestant Nonconformists.

As for the word sect, the original 14th-century sense of this word referred to a dissenting religious body, particularly one that is heretical in the eyes of other members within the same communion. Sect is derived from the Latin secta, which means "way of life" or "school of thought" and comes from the Latin verb sequi, meaning "to follow."