Is it ensure, insure, or assure?
1590s, "one who sneaks," agent noun from sneak (v.). Meaning "rubber-soled shoe" is attested from 1895, American English; earlier sneak (1862), so called because the shoe was noiseless. See also plimsoll.
The night-officer is generally accustomed to wear a species of India-rubber shoes or goloshes on her feet. These are termed 'sneaks' by the women [of Brixton Prison]. ["Female Life in Prison," 1862]