From stammer +‎ -er.

stammerer (plural stammerers)

1. One who stammers; a stutterer.
   • 1833, [Charles Lamb], “Preface. By a Friend of the Late Elia.”, in The Last Essays of Elia.[…], London: Edward Moxon,[…], →OCLC, page ix:

     Marry—as the friendly vapour ascended, how his prattle would curl up sometimes with it! the ligaments, which tongue-tied him, were loosened, and the stammerer proceeded a statist!