Two short lines, and a grim joke tucked between them. The first sounds like a motto for enjoying life: to dine well is to live. Good food, good company, pleasure in the body. Then the second line turns it around. "Fit" usually means healthy and in shape, but here the speaker says they will be "fit as an urn," and an urn is what holds a person's ashes.
That twist changes how we read the first line. Maybe the speaker is saying that indulgence leads, cheerfully, to an early grave. Or maybe they're pointing out that death is the final shape we all end up in, whatever we eat. The tone is dry and playful, a little defiant, with a shrug at mortality. The speaker seems to accept the end without fuss, and still insists on enjoying the meal. The poem's brevity makes the punchline land hard: pleasure and death sit side by side, almost sharing a plate.