"capable of being used in place of another; capable of being replaced," 1818, a word in law originally, from Medieval Latin fungibilis, from Latin fungi "perform" (see function (n.)) via phrases such as fungi vice "to take the place." Earlier as a noun (1765).

*bhrūg-, Proto-Indo-European root meaning "to enjoy," with derivatives referring to agricultural products.

It forms all or part of: brook (v.) "to endure;" defunct; fructify; fructose; frugal; fruit; fruitcake; fruitful; fruition; fruitless; frumentaceous; function; fungible; perfunctory; tutti-frutti; usufruct.

It is the hypothetical source of/evidence for its existence is provided by: Latin frui "to use, enjoy," fructus "an enjoyment, proceeds, fruit, crops;" Old English brucan "use, enjoy, possess," German brauchen "to use."