"Secure" derives from the Latin words se, meaning "without", and cura, meaning "care" or "worry". In simplest terms, being secure means not having to worry about security. Threats to well-being are just not of concern.
This is important to note as most people use the term "secure" to mean a variety of different things. There's some people who use "secure" interchangeably with the elements from the CIA Triad. There are some who mean that an asset is important and adequately protected. And there are those who even use "secure" to mean "comfortable", as in "I am secure with who I am". What's surprising is how close the last use really is to its etymological roots. There are even those who use "secure" to mean physically fastened to another object. Personally, I am most frustrated with people who use the word "secure" or "security" interchangeably with, say, Confidentiality, because it shows a flagrant ignorance into the bigger picture of what it takes for, say, users of a system to be comfortable that their information assets are truly safe. Clearly, more is needed than just Confidentiality, hence all of these models like the CIA Triad and Parker's Hexad. [For the record, I think both of those models are inaccurate, but that's for another time.]