Is it ensure, insure, or assure?
Old English stamerian, from West Germanic *stamrojan (cf. Old Norse stammr, Old High German stam, Gothic stamms "stammering," Middle Dutch stameren, German stammeln "to stammer," Old Frisian and German stumm "dumb"), from PIE root *stam-, *stum- "check, impede" (see stem (v.)).
stammer stam·mer (stām'ər)
A speech disorder characterized by hesitation and repetition of sounds, or by mispronunciation or transposition of certain consonants, especially l, r, and s. v. stam·mered, stam·mer·ing, stam·mers
To speak with a stammer.