As far as language goes, words that describe what's in style at the moment tend to ebb and flow. "Groovy" has fallen by the wayside, and "rad" has become retro irony, uttered by hipsters in bars who might not have even been alive when the term peaked in the 1980s.
But the word "cool" has persevered. Ansgar Kelly, a research professor in English at UCLA, said its staying power lies partly in the fact that it has roots in an entirely different meaning.
"It is a real word," he said. "'Cool' is always with us; it keeps its original meaning, which is that you're trying not to get hot. It means that you are keeping your cool; you are sophisticated."
Words that become trendy but don't already exist in the English language, he said, have "a limited shelf life," and "it's hard to give a hard and fast rule" as to whether they'll live on beyond a few years.