What is the X in X-mas?
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 cf. 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.