"Fell headlong" refers to falling abruptly and quickly with one's head leading the way. Some synonyms for "fell headlong" may include "tumbled", "plunged", "toppled", "dropped", "collapsed" or even "dove". These terms can be used interchangeably depending on the context of the sentence, and add diversity to one's writing. For example, instead of using "fell headlong" repeatedly, a writer may opt to use a synonym like "tumbled" to describe the same action. By using synonyms, writers can enhance their creativity and bring their writing to life in a unique way.