Full Idea
It seems likely that the content of the word 'true' is sui generis and indefinable
Clarification
'Sui generis' means of a unique kind
Gist of Idea
The word 'true' seems to be unique and indefinable
Source
Gottlob Frege (The Thought: a Logical Enquiry [1918], p.327 (60))
Book Reference
Frege,Gottlob: 'The Frege Reader', ed/tr. Beaney,Michael [Blackwell 1997], p.327
A Reaction
This is the view I associate with Davidson, though fans of Axiomatic Truth give up defining it, and just describe how it behaves. Defining it is very elusive, but I don't accept that nothing can be said about the contents of the concept of truth.