What is the etymology of the word "secular"?

secular (adj.)
c.1300, “living in the world, not belonging to a religious order,” also “belonging to the state,” from Old French seculer (Modern French séculier), from Late Latin saecularis “worldly, secular, pertaining to a generation or age,” from Latin saecularis “of an age, occurring once in an age,” from saeculum “age, span of time, generation.”

According to Watkins, this is probably from PIE *sai-tlo-, with instrumental element *tlo + *sai- “to bind, tie” (see sinew), extended metaphorically to successive human generations as links in the chain of life. Another theory connects it with words for “seed,” from PIE root *se- “to sow” (see sow (v.), and compare Gothic mana-seþs “mankind, world,” literally “seed of men”).

Used in ecclesiastical writing like Greek aion “of this world” (see cosmos). It is source of French siècle. Ancient Roman ludi saeculares was a three-day, day-and-night celebration coming once in an “age” (120 years). In English, in reference to humanism and the exclusion of belief in God from matters of ethics and morality, from 1850s.

http://www.etymonline.com/index.php?term=secular

From the OED, which would seem to agree:

“Etymology: In branch I, < Old French seculer (modern French séculier ), < Latin saeculāris , < saeculum generation, age, in Christian Latin ‘the world’, especially as opposed to the church: see secle n., siècle n. In branch II, directly < Latin saeculāris, whence modern French séculaire (which has influenced some of the uses in English). Compare Spanish seglar, secular, Portuguese secular, Italian secolare.”

Thanks.

I understand your confusion. I always get confused between secular and nonsecular on the one hand and sectarian and nonsectarian on the other.

Here is my system for remembering. Sectarian contains sect, so it must refer to religion. Nonsectarian is therefore the opposite. Then I recall that secular and nonsecular go in the reverse order.

There may be a connection between the Latin and ancient Hebrew. Segula is an concept (based on Kabbala) meaning treasure, or magic, or something otherworldly. See, among other things, link and link

Magical charms such as these are/were somewhat of a departure of traditional religion, and therefore might be seen as non-religious.

You might ask how the Hebrew S G L root would become Latinized to “secula” or S C L…that is the way that languages get bastardized over centuries.

When it’s used within the context of religion, i.e., the Catholic Church, it refers to a priest who is not affiliated with any order, e.g., Franciscan, Jesuit, etc., and is responsible to the hierarchy of the parish or diocese in which he serves. In a broader sense, it refers to things not affiliated with any religion, as in secular humanist.

From the OED:

Etymology, n.

The facts relating to the origin of a particular word or the historical development of its form and meaning; the origin of a particular word.