1530s, "without care, dreading no evil," from Latin securus, of persons, "free from care, quiet, easy," also in a bad sense, "careless, reckless;" of things, "tranquil; free from danger, safe," from *se cura, from se "free from" (see secret (n.)) + cura "care" (see cure (n.)).
In English, of places, "free from danger, unexposed," from 1580s. Meaning "firmly fixed" (of material things) is from 1841, on notion of "affording grounds for confidence." Of telephones, "not wiretapped," from 1961. Replaced Middle English siker, from Old English sicor, from the Latin word. Related: Securely.