The phenomenon that, in a film adaptation of a book, certain characters are portrayed as a lot more stupid than they are in the book. Supposedly this is done to make the movie 'more entertaining', but it often annoys fans of the book.
The name is derived from Merry and Pippin, two characters in The Lord of the Rings
who are a helluvalot more stupid in the film than they are in the books.
That Zaphod Beeblebrox is really suffering from the Merry-and-Pippin syndrome.