Green’s Dictionary of Slang

dinner jacket.

1959		A. Sinclair My Friend Judas (1963) 113: I slipped straight into my gadabouts, double-breasted D.J., white shirt with button-downs.
1980		A. Payne ‘All Mod Cons’ Minder [TV script] 66: Terry takes his D.J. off and hurls it across Filmer’s desk.
1985		A. Payne ‘Minder on the Orient Express’ Minder [TV script] 28: Terry, in D.J., sits with his feet up reading a paper.