mys·te·ri·ous |məˈsti(ə)rēəs|

adjective

1. difficult or impossible to understand, explain, or identify : his colleague had vanished in mysterious circumstances | a mysterious benefactor provided the money.
   • (of a location) having an atmosphere of strangeness or secrecy : a dark, mysterious, windowless building.
2. (of a person) deliberately enigmatic : she was mysterious about herself but said plenty about her husband.

DERIVATIVES

mys·te·ri·ous·ly |məˈstɪriəsli| adverb

mys·te·ri·ous·ness |məˈstɪriəsnəs| noun

ORIGIN late 16th cent.: from French mystérieux, from mystère ‘mystery.’