Is it ensure, insure, or assure?
Old English bur "room, hut, dwelling, chamber," from Proto-Germanic *buraz (cf. Old Norse bur "chamber," Swedish bur "cage," Old High German bur "dwelling, chamber," German Bauer "birdcage"), from *bu- "to dwell," from PIE root *bheue- "to be, exist, dwell" (see be). Modern spelling developed after mid-14c. Sense of "leafy arbor" (place closed in by trees) is first attested 1520s. Hence, too, Australia's bower-bird (1847).