proverb/le plɛ.zɑ̃.tʁi le ply kuʁ.t(ə) sɔ̃ le mɛ.jœʁ/
your joke was funny at first, but it's getting old; that's enough, you're starting to flog a dead horse now
Literally, “the shortest jokes are the best”, or more loosely translated as “the best jokes are the shortest”.