Is it ensure, insure, or assure?
1830, shortening of spermaceti whale (so called because the waxy substance in its head was mistaken for sperm), from spermaceti (1471), from Medieval Latin sperma ceti "sperm of a whale," from Latin sperma (see sperm) + cetus "large sea animal" (see Cetacea). The substance in olden times was credited with medicinal properties, as well as being used for candle oil.
Use ... Sperma Cete ana with redd Wyne when ye wax old. [Sir George Ripley, "The Compound of Alchemy," 1471]