Since 2015 there has been a phrase of Swiss origin in the Larousse French dictionary to express one's sense of pleasant surprise – the idea that one was expecting the worst but somehow avoided an awful experience.
‘Déçu en bien’ when translated directly means, rather confusingly, "disappointed in good".
One might use it when expecting a bad meal or a terrible film having previously read a review, only to be pleasantly surprised in the end.
Read also: La go, brouteur: Do you know these French words in Petit Robert 2023?
Understated praise
It is, of course, an example of speaking ironically – in the same way as one might say "being economical with the truth" (s'arranger avec la vérité) when referring to a liar.
It is also a good example of expressing understated, incomplete or not wholly sincere praise – just as we would "damn someone with faint praise".
The French here might refer to the speaker as ‘faux-cul’ (literally "fake arse"), meaning a hypocrite.
Root of the phrase
At the root of ‘déçu en bien’ is the verb décevoir (to disappoint).
The noun la déception (disappointment) is one of the most notably tricksy examples of what are known as 'false friends' (faux amis) to English speakers learning French – words that look or sound like English ones but which have a completely different meaning in French.
Deception (English), meaning fraudulence, dishonesty or misleading words or behaviour is actually called la tromperie in French, and is commonly employed to describe the acts of cheating on one's partner, husband or wife.
Interestingly, the ancient origin of tromper is from jouer de la trompe (playing the trumpet) which, over the centuries, evolved to mean ‘playing someone’.
Related articles
Four French words to fill gaps in vocabulary that mean 'the thing'
Hein? A little word that helps you sound much more French
Five reasons to learn French – and why they may help you pick it up
Visa and residency cards for France*
Inheritance Law and Wills in France* 
