A simple dictionary definition of fungible is “replaceable by another identical item; mutually interchangeable”. I find that an example and a comparison is more helpful. Money is fungible. A £20 note can be used to buy anything up to that value. Time is not fungible. The time that you spend commuting to work is not the same as the time that you spend at work itself.
As with “interstitial” (a Word of the Week from last month) I first came across fungible in an Oliver Burkeman article, and think of it in relation to time. I recommend that you follow this link and read his piece (from August 2011) so that I don’t have to paraphrase it. The time that I spend paraphrasing his words is time that could be spent doing something else.
The word appears in 5 books that I have on my Kindle, all in the realms of “Smart Thinking” (or Self Help, or personal productivity, or whatever you prefer to call it), so I have been able to look them up easily. It’s in Oliver Burkeman’s “Help! How to feel slightly happier and get a bit more done”, which grew out of his regular “This column will change your life” articles, and it’s in Jonah Lehrer’s “The Decisive Moment”. In both books it appears once, in the context of money. “Are you smart enough to work at Google?” by William Poundstone mentions “fungible creativity”, and “I used to know that: English (stuff you forgot from school)” by Patrick Scrivenor quotes Sarah Palin talking about oil and coal as “a fungible commodity”. Finally, Philip Zimbardo’s “The Time Paradox” mentions it in the context of time: “You can’t bottle time and exchange it for an object or event”.
It’s a word that I have never heard in conversation, only read it in books and articles, and, from what I can tell, the “g” is soft, as in “tangible”, so it’s pronounced “fun-jibble”. If it comes up in conversation, or in a movie, TV or radio show, I’ll let you know.