Full Idea
The predicate 'is a prime number greater than all other prime numbers' is necessarily not true of anything, but it is not semantically defective, for it occurs in sentences that constitute a sound proof that there is no such number.
Clarification
It is proven that there is no greatest prime number
Gist of Idea
References to the 'greatest prime number' have no reference, but are meaningful
Source
Timothy Williamson (Vagueness [1994], 6.2)
Book Reference
Williamson,Timothy: 'Vagueness' [Routledge 1996], p.168
A Reaction
One might reply that the description can be legitimately mentioned, but not legitimately used.