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, lifetime, generation, breed."
This is from Proto-Italic *sai-tlo-, which, according to Watkins, is PIE instrumental element *-tlo- + *sai- "to bind, tie" (see sinew), extended metaphorically to successive human generations as links in the chain of life. De Vaan lists as a cognate Welsh hoedl "lifespan, age." An older theory connected it to 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 aiōn "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.