who ‧ dun ‧ it BrE AmE , whodunnit /ˌhuːˈdʌnət, ˌhuːˈdʌnɪt/ noun [countable] informal
[ Date: 1900-2000 ; Origin: who done it? 'who did it?' ]
a book, film etc about a murder case, in which you do not find out who killed the person until the end
who ‧ dun ‧ it BrE AmE , whodunnit /ˌhuːˈdʌnət, ˌhuːˈdʌnɪt/ noun [countable] informal
[ Date: 1900-2000 ; Origin: who done it? 'who did it?' ]
a book, film etc about a murder case, in which you do not find out who killed the person until the end
Longman Dictionary of Contemporary English. Longman - Словарь современного английского языка. 2012