Brewer's: Damn with Faint Praise
To praise with such a voice and in such measured terms as to show plainly secret disapproval.
Damn with faint praise, assent with civil leer, And, without sneering, teach the rest to sneer.
Pope: Epistle to Arbuthnot.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894