The title, a French phrase for the clever reply you think of only after the moment has passed, gives the key to this short poem. The speaker keeps going back over conversations, chewing on words already spoken. "Batten like a mad king" suggests a greedy, slightly unhinged appetite, as if replaying the past were a feast he can't stop himself from eating.
The quoted line, "The memory of the wicked shall rot," sounds like a biblical warning, and the speaker calls it his "text," as in a sermon's text. It reads as a harsh judgment on himself, or on the kind of memory he indulges in. Then the poem turns eerie. After he leaves, he "makes sport / in imitating the dead," acting out what he should have said, ghostlike, "grown fat on moonlight." The mood is nocturnal, guilty, and darkly comic. It captures the lonely, self-mocking feeling of lying awake rehearsing arguments nobody will ever hear.