Callithump \KAL-uh-thuhmp\ , noun;
1. A children's parade, with prizes for the best costumes
Callithump is a back-formation of callithumpian, which is a colloquial word that is probably a fanciful formation dating to 1836. It may be related to gallithumpians ("a society of social reformers" or "disturbers of order at Parlimentary elections").
I realize that this will be the third* time I've referred to a 'back-formation' in a post, but I haven't explained what that is.
'Back-formation' is the creation of a new word (lexeme, technically) by removing affixes. The affixes can be real or assumed, depending on the word. From this process, editor begat edit, beggar begat beg, and many more.
* See dapple and gastronome.
Today's word and the first definition were both taken from Dictionary.com's 'Word of the Day' for Saturday, May 7
Etymologies come from the Oxford English Dictionary and/or Etymonline.com