The speaker is fed up with a man whose words do damage wherever he goes. The poem opens with a question, "how is a foul mouth to be remedied", and the tone is half weary, half exasperated, as if the speaker doesn't really expect an answer. The mouth is "fit for every war and after-dinner blow-up", which suggests the talker is as comfortable starting fights at a dinner table as he would be in a larger conflict. Petty and serious quarrels seem to blur together.
Then the mood turns harsher. "Out with his absent truth" reads like a demand to throw him out, and it also hints that his talk is empty of honesty. The final image, a "white fish-belly / big as a blemish on the night", is unflattering and a bit disgusting. It makes the man pale, bloated and out of place, a stain on the dark. The poem feels short-tempered and disgusted, with a dry, bitter humor running through it.