Is it ensure, insure, or assure?
Old English bece "beech," from Proto-Germanic *bokjon (cf. Old Norse bok, Dutch beuk, Flemish boek, Old High German buohha, German Buche, Middle Dutch boeke "beech"), from PIE root *bhagos "beech tree" (cf. Greek phegos "oak," Latin fagus "beech," Russian buzina "elder"), perhaps with a ground sense of "edible" (and connected with the root of Greek phagein "to eat;" see -phagous). Beech mast was an ancient food source for agricultural animals across a wide stretch of Europe. Formerly with adjectival form beechen. Also see book.