sinew n.

1640s, "to disable, render useless," a figurative verbal extension from hamstring (n.) "tendon at the back of the knee." Cutting this would render a person or animal lame. The literal sense of the verb is attested from 1670s. Because it is a verb from a noun-noun compound, hamstrung as its past participle is technically incorrect.

[I]n hamstring, -string is not the verb string; we do not string the ham, but do something to the tendon called the hamstring; the verb, that is, is made not from the two words ham & string, but from the noun hamstring. It must therefore make hamstringed. [Fowler]

An older term for the same thing was hough-sineuen (15c.), with hock (n.1) + sinew (n.), also a noun-noun compound.

c. 1300, seculer, in reference to clergy, "living in the world, not belonging to a religious order," also generally, "belonging to the state" (as opposed to the Church), from Old French seculer, seculare (Modern French séculier) and directly from Late Latin saecularis "worldly, secular, pertaining to a generation or age," in classical Latin "of or belonging to 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 also connects it with "bind" words and 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").

The ancient Roman ludi saeculares was a three-day, day-and-night celebration coming once in an "age" (120 years).  Ecclesiastical writers in Latin used it as those in Greek did aiōn "of this world" (see cosmos). It is the source of French siècle "century." The meaning "of or belonging to an age or a long period," especially occurring once in a century, was in English from 1590s.

From mid-14c. in the general sense of "of or belonging to the world, concerned in earthly more than in spiritual, life;" also of literature, music, etc., "not overtly religious." In English, in reference to humanism and the exclusion of belief in God from matters of ethics and morality, from 1850s. Related: Secularly.

late 14c., "made of sinews" (a sense now obsolete), from sinew + -y (2). As "brawny, muscular, robust," from early 15c. As "tough, stringy" from 1570s.