Merriam-Webster Unabridged gives two definitions of fungible, but here is the one that Dogbert has in mind: "of such a kind or nature that one specimen or part may be used in place of another specimen or equal part in the satisfaction of an obligation." The entry goes on to explain that fungible is usually used of things that can be counted, weighed, or measured, such as food, coal, oil, lumber.

Fungible also has an extended sense, meaning "capable of mutual substitution; interchangeable." And the Eleventh Edition of the Collegiate Dictionary adds one more sense: "characterized by a ready capability to adapt to new, different, or changing requirements; flexible."

Fungible is not the most common of words, but it does appear regularly in many widely read publications. Here is a sampling from the Merriam-Webster citation file.

...the USDA wants you to believe that two to three ounces of cooked lean meat, poultry, or fish, one cup of cooked dry beans, two eggs, or four tablespoons of peanut butter are fungible quantities – each is equivalent to one serving. — Raymond Sokolov, Natural History, January 1994

Payroll taxes go directly to today's beneficiaries; benefits come directly from today's workers. Since FICA is a tax, and tax revenues are fungible, any annual surplus of FICA taxes over benefits is used to cover other government spending. — Peter G. Peterson, Atlantic Monthly, May 1996

Back in Washington he sought an outlet for his talents, which were not exactly fungible. He declined an offer from a former C.I.A. colleague, E. Howard Hunt, to participate in the Watergate burglary. — Stanley Karnow, New York Times Magazine, January 3, 1999

At a time when so many rituals and civic experiences have lost that sense of moment, when wedding vows have become increasingly fungible and voting has been devalued by the dangling chads of Florida ..., service on a jury remains perhaps the only public service that ... still has the power to elevate an ordinary citizen. — Anna Quindlen, syndicated column, May 06, 2001

Fungible derives from the Latin verb fungi, meaning "to perform" and whose past participle is functus, from which we get the English words defunct, function, and perfunctory.