Is it ensure, insure, or assure?
mid-14c., "eager to know" (often in a bad sense), from Old French curios "solicitous, anxious, inquisitive; odd, strange" (Modern French curieux) and directly from Latin curiosus "careful, diligent; inquiring eagerly, meddlesome," akin to cura "care" (see cure (n.)). The objective sense of "exciting curiosity" is 1715 in English. In booksellers' catalogues, the word means "erotic, pornographic." Curiouser and curiouser is from "Alice in Wonderland" (1865).