also fruit-cake, 1838 in the literal sense "a rich, sweet cake containing fruit," from fruit + cake (n.). Slang meaning "lunatic person" is first attested 1952.

*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."