Is it ensure, insure, or assure?
mid-14c., from Old French diligence "attention, care; haste, speed," from Latin diligentia "attentiveness, carefulness," from diligentem (nominative diligens) "attentive, assiduous, careful," originally present participle of diligere "single out, value highly, esteem, prize, love; aspire to, be content with, appreciate," originally "to pick out, select," from dis- "apart" (see dis-) + legere "choose, gather" (see lecture (n.)).
Sense evolved from "love" through "attentiveness" to "carefulness" to "steady effort." From the secondary French sense comes the old useage of diligence for "public stage coach" (1742; dilly for short), from a French shortening of carrosse de diligence.
large, four-wheeled, closed French stagecoach employed for long journeys. It was also used in England and was popular in both countries in the 18th and 19th centuries.