Is it ensure, insure, or assure?
1738, "the highest singing voice," ranging easily through the two octaves above middle C, from Italian soprano "the treble in music," literally "high," from sopra "above," from Latin supra, fem. ablative singular of super (see super-). Meaning "a singer having a soprano voice" is from 1738. As an adjective from 1730. Soprano saxophone is attested from 1859.