Full Idea
There are practically no defensible examples of definitions; for all the examples we've got, practically all the words (/concepts) are undefinable.
Gist of Idea
We have no successful definitions, because they all use indefinable words
Source
Jerry A. Fodor (Concepts:where cogn.science went wrong [1998], Ch.3)
Book Reference
Fodor,Jerry A.: 'Concepts: where cognitive science went wrong' [OUP 1998], p.45
A Reaction
I don't think a definition has to be defined all the way down. Aristotle is perfectly happy if you can get a concept you don't understand down to concepts you do. Understanding is the test, not further definitions.