means the quality of holding firmly to a goal, grip, or position and refusing to let go despite difficulty.

from From Latin tenax, "holding fast," which grows straight out of the verb tenere, "to hold" — the same root clutching at words like tenant (one who holds land), tenable (able to be held), and tenacious. It reached English through French in the 1500s, carrying that original sense of a grip that won't release.