stammer

Short definition from Wiktionary

Verb

1. 1
   To keep repeating a particular sound involuntarily during speech.
2. 2
   To utter with a stammer, or with timid hesitancy.

Noun

1. 1
   The involuntary repetition of a sound in speech.
2. 2
   A speech defect whereby someone speaks with a stammer

Proper noun

1. 1
   A surname from German.