My dad said he was going to dress up as a giant grape for Halloween.

This joke is a classic example of wordplay, specifically a pun, that hinges on a clever sound-alike. The humor comes from the unexpected twist at the end, where the word "raisin" is used in place of "raising," as in "raising the spirits" or "raising the roof" at a party. Because the dad is dressed as a giant grape, the idea of him becoming a "raisin" – which is simply a dried grape – is a perfectly logical, albeit comically deflating, outcome for his costume.

Halloween is often a time for silly and unexpected costumes, and a giant grape certainly fits the bill for lighthearted absurdity. The setup plays on the common idiom "the life of the party," which refers to someone who brings energy and fun to a social gathering. The punchline subverts this expectation by taking the literal form of a dried-up grape, a raisin, which is decidedly less energetic than a fresh, plump grape. It's a delightful little linguistic trick that combines costume logic with a well-placed homophone for a simple, sweet laugh.