Is it ensure, insure, or assure?
mid-13c., "cloth of reddish-brown color," also (early 15c.) the color of this, from Old French rousset, from rosset (adj.) "reddish," diminutive of ros, rous "red," from Latin russus, which is related to ruber "red," from PIE *reudh- "red" (see red (adj.1)). As an adjective from late 14c. The word was applied to a type of apples first in 1620s, to a type of pears 1725.