A flimsy excuse if ever I heard one.
The other day, I was explaining the meaning of "if ever I heard one" to my colleague, a native French speaker. The phrase, if rather literally translated, seems to mean:
Voilà une piètre excuse, si tant est qu'il y en ait une.
... but this French expression is premised on the assumption that it is unlikely, if not impossible, that such a thing as "a flimsy excuse" exists in the first place — which is an illogical notion, if ever there was one!
The English phrase "if ever I heard one", on the other hand, is more like hyperbole, singling out this particular excuse as the most flimsy one imaginable among all others.
So I explained that one of the closest translations would be:
Voilà une piètre excuse, s'il en est.
But I'm not completely sure about all of this. I wonder if "si tant est qu'il y en ait une" works as well, after all?