As a musical direction: in a mysterious manner; in a manner suggestive of mystery.
1An air of mystery; mysteriousness. Also: a film, television programme, etc., whose story relies on mystery and suspense.
A trick position adopted by a surfer.
Of music: played misterioso. Also more generally: mysterious.
Late 19th century; earliest use found in John Stainer (1840–1901), musicologist and composer. From Italian misterioso mysterious (also used as a performing direction in music).