Is it ensure, insure, or assure?
"cunning," early 14c. alteration of sleahthe (c.1200), from Old Norse sloegð "cleverness, cunning, slyness," from sloegr (see sly). Meaning "skill, cleverness, dexterity" is from late 14c. Meaning "feat or trick requiring quickness and nimbleness of the hands" is from 1590s. Term sleight of hand is attested from c.1400.